User-defined attributes in DWG and DXF files
AutoCAD provides several methods to store user-defined attribute data in DWG and DXF files.
A DWG block attribute is a user-defined textual entity used for storing data. A block attribute is a nested member of an AutoCAD block definition and can be configured to be visible or not visible. In addition to attribute entities, a block reference often includes graphic elements, such as lines or point entities used as symbology.
ArcGIS displays block attributes as fields in the CAD feature class attribute table. The block attribute tag name defined in AutoCAD is translated as the fieldname. AutoCAD implicitly defines all block attributes as a text data type. ArcGIS assigns a data type that is appropriate for the contents of the field.
Xrecords and object dictionaries
AutoCAD xrecords are nongraphic container objects used for storing data in a DWG file. Object dictionaries are used to store xrecords and can be assigned to the drawing or linked directly to an entity's extension dictionary.
ArcGIS uses an integrated implementation of AutoCAD xrecords and object dictionaries with the Export To CAD tool to extend support for storing feature class definitions and attributes attached to AutoCAD entities.
This geodata is nongraphic and does not interfere with standard AutoCAD geometry or native functionality. The AutoCAD xrecord objects that define the feature classes and coordinate system are stored in a standardized framework of AutoCAD object dictionaries. Similarly, entity-linked attributes are stored in the AutoCAD extension dictionary referenced by each entity.
For detailed information about how this data is structured, reference the ESRI document Mapping Specification for CAD available on the ArcGIS for AutoCAD Resource Center.
Extended entity data
AutoCAD extended entity data (xdata) is used to attach non-graphic attribute information to AutoCAD entities. ArcGIS 10 replaces using xdata with an automated implementation of AutoCAD xrecords that is integrated with the Export to CAD tool. The tools that support xdata remain in the system so that your existing models and scripts will continue to work, but no further development will be performed in future releases.