About the image service definition file
Legacy:ArcGIS 10 is the last release of the stand-alone ArcGIS Image Server product. The image service definition (.ISDef) has been replaced by an improved geodatabase data model—the mosaic dataset—which can be published as an image service using the ArcGIS Server Image extension.
The image service definition is used to define an image service, including the data, processing, rendering, metadata, extent, and properties. It is created with the ArcGIS Image Server extension using the Image Service Definition Editor toolbar in ArcMap.
Learn about the Image Service Definition Editor toolbar
Note:If you are creating a new image service definition, it is recommended that you consider creating a mosaic dataset instead. The mosaic dataset is an improved data model for serving content similarly to an image service definition. To learn more, see What is a mosaic dataset.
The image service definition exists as a folder with the extension .ISDef. Within the folder is the ImageService.ISDef file that is an Extensible Markup Language (XML) file containing the image service properties. There can also be several shapefiles used to define the raster dataset footprints, the boundary of the raster datasets, and any seamlines. There is an .RPDef folder within the .ISDef folder. This holds the raster process definition files that define each raster dataset and any processes to be applied directly to them.
Learn more about the image service definition file structure
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In ArcMap, the image service definition appears as a custom layer in the table of contents, containing footprint, boundary, seamline, and preview layers. The boundary and preview layers are displayed after raster data is added to the image service definition and the image service definition is built. The seamline layer is only available if seamlines have been generated for the image service definition.
Service properties
You can view the image service properties on the Image Service Properties dialog box. This is available either by clicking the Image Service Properties button 
on the Image Service Definition Editor toolbar or by right-clicking the image service definition layer in the ArcMap table of contents and clicking Properties.
The image service definition properties define the properties of an image service including the parameters, such as the number of bands and pixel type of the raster data to be mosaicked, metadata about the service, processes to be applied to the mosaicked raster data, defaults for client-defined service properties, and defaults to be used by different features of the Service Editor.
Service table
The service table acts as a catalog of the raster datasets that make up an image service definition. It is stored as part of the footprint shapefile within an image service definition. You can open the service table either by clicking the Service Table button 
on the Image Service Definition Editor toolbar or by right-clicking the image service definition layer in the ArcMap table of contents and clicking Open Service Table.
Each record in the table includes a pointer to the raster dataset source and basic information about the raster data, such as the type of raster, an ID number, and information about the pixel sizes. The service table also contains Status ID and Status Message fields that are used to provide feedback if there are problems with a specific raster dataset. The service table may also contain metadata attributes for each raster dataset that can be used to order the raster datasets when they are mosaicked or as attributes required by a service administrator to manage the large number of raster datasets that make up a single table.
Footprint
The footprint contains the outline of the raster dataset. This outline is not always the extent of each raster dataset but can be the extent of the valid raster data within the dataset. For example, in some satellite imagery, such as Landsat, the extent of raster data is not rectangular as suggested by the file extents. It is, in fact, contained within a skewed rectangle in the dataset, and the other pixel values surrounding this skewed rectangle are null data. The example below displays a Landsat 1G scene and the footprint that would be derived.
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The footprint is generated based on the type of raster dataset chosen while adding raster data to the image service definition. For example, when you choose to add a TIFF file with a world file, it creates a footprint for the entire raster dataset. On the other hand, when you choose to add a Landsat file, it generates a footprint without the null data.
Each raster dataset is clipped by the footprint as it is mosaicked. The clipping enables the image service to exclude pixels that are not to be displayed. The use and clipping based on footprints is more efficient than using transparent pixels. Areas of NoData are a typical example, but the footprint can also be used to clip off the collars of scanned maps or remove the black frame and fiducial marks around georeferenced, scanned aerial photography.
The footprint for each raster dataset is saved as a polygon shapefile in the .ISDef folder. Since the footprint is a shapefile, it can be edited using the standard ArcMap feature editing tools.
Learn more about editing footprints
In the image below, the footprint for each overlapping raster dataset is displayed in green, with a single raster dataset footprint selected and highlighted in cyan.
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Boundary
The boundary outlines the extent of all the datasets as a whole. It is generated from the footprints of the primary raster datasets when the image service definition is built. Derived raster datasets, such as service overviews and raster tiles, are not used to generate the boundary. The service boundary is used to enable a client application to correctly identify areas covered by imagery, which is especially useful when working with different projections. To ensure that the boundaries do not contain too many vertices and become too large, they can be generalized. The boundaries are generated by the Service Editor during the process of building an image service definition. The boundary of an image service is stored as a shapefile in the .ISDef folder and can be edited using standard ArcMap tools. In the image below, the boundary is shown in pink and has been generated for the raster datasets that were used to generate the footprint example (above).
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Preview
The preview layer enables the image service definition to be viewed within ArcMap without having to compile and publish the service. Using the preview, you can review the service and check the correctness of the processing and service properties prior to compilation. A preview is only available after the image service definition has been built. Since the image from the preview layer is not created by the server, the speed of image viewing is not as fast as that available from a compiled service.
Seamline
Seamlines are used to define the extents of each raster dataset, rather than the footprints, when you define seamline mosaic as the mosaicking method. When you use seamline mosaic, the raster datasets are mosaicked along boundaries, with the option of including feathering. Feathering blends the images together smoothly along the seamlines so that there are no sharp transitions between images.
Seamlines are generated by exporting footprints and editing them or by examining the overlap in the raster datasets and intelligently creating a seamline to follow natural patterns between the overlap. Seamlines can also be directly imported as part of a raster dataset if the raster type defines them. Seamlines are stored as a shapefile in the image service definition (.ISDef) folder and can contain relational links to the footprint and service table. The seamline can be edited using standard ArcMap tools.
The compiled image service definition
When you have set all the parameters for the image service definition and built the image service definition to compute additional service parameters, it can be compiled using the Compile operation. Compiling the image service definition encapsulates its contents into a single, compact file with an .ISCDef extension and includes specially optimized indexes, which are required to enable ArcGIS Image Server to quickly access the required raster data. The .ISCDef file is published to serve the image service to clients.