Layer definitions provide a means of delivering customizations that are loaded with data. Typically, this includes data entry forms with associated scripts to handle data entry validation and other features offered on the forms. The layer definition can also contain symbology exported from ArcGIS for Desktop or ArcView 3.x.
For shapefiles, layer definitions are stored in a file associated with the shapefile. This file has the same filename as the shapefile, but has a .apl extension. The layer definition file lives and travels with the shapefile. For ArcPad AXF files, the layer definition is stored in a table in the AXF file.
Create layer definitions
The forms and scripts portion of layer definitions are created in ArcPad Studio. The symbology portion is created in ArcGIS for Desktop using the Get Data for ArcPad tool, or in ArcView 3.x using the Export ArcPad Layer Symbols tool. These two procedures can be done in either order. The layer definition is stored in ArcPad Extensible Markup Language (XML) format.
Once the framework of a layer definition has been defined in ArcPad Studio using the Get Data for ArcPad tool or Export ArcPad Layer Symbols tool, you can use any text or XML editor to make changes or additions. In fact, it's possible to create the entire layer definition using only a text or XML editor, although it's generally inefficient to do so.
Load layer definitions
ArcPad automatically loads layer definitions when the associated layers are loaded.