Copied straight out of the help area (so I can find this again when I need it)
3. Object Data commands available
Currently there are only five special commands in AutoCAD Map 3D to work with Object Data:
1. _ADEDEFDATA to create and edit Object Data descriptions.
This command is used to create Object Data descriptions composed of OD name that is unique within the DWG file (up to 25 characters) and value fields of type integer, char, point or real. The fields are defined with unique within one OD (up to 31 characters) names, descriptions (up to 40 characters) and default values (up to 132 actual characters though character fields are defined as 254 characters after export).
Cyrillic characters can be used in OD names and OD fields.
There is no limit of Object Data descriptions in DWG file. The number of fields in one OD is also unlimited. Real files observed to contain more than a hundred OD descriptions and more than thirty fields in one OD.
This command also allows Object Data descriptions editing – rename OD, remove descriptions, remove fields, change field descriptions and default values, add new fields.
2. _ADEATTACHDATA to attach/detach Object Data to/from graphic elements.
This command is used to attach Object Data to selected graphic elements, remove attached Object Data and define field values.
Several OD including identical can be attached to one graphic element.
The option of the identical OD overwriting is available, i.e. one can choose whether to remove the previous OD record or to create a duplicate.
The allowed number of OD per graphical element is not determined.
3. _ADEEDITDATA to edit and add/remove similar Object Data.
This command makes it possible to browse the Object Data attached to the specified graphic element, examine and edit the field values. It is also possible to add similar OD attached to this graphic element, remove OD, browse all duplicate OD and edit field values.
4. _MAPDIGISETUP to setup drawing with simultaneous Object Data attachment.
This command is used to determine a graphical element entity class (block, polyline, 3D polyline) to draw, the layer for this element, name and insert options for blocks, width and line type for polylines, the Object Data immediately attached to the element.
This command only partially refers to Object Data commands because Object Data attachment to the drawn elements is optional.
This command makes it possible to describe only one parameter set for drawing of single type of graphic element with simultaneous attachment of corresponding Object Data at a time. In other words, when describing a new drawing parameter set with OD the previous set is deleted. Consequently, it is impossible to create objects menu for drawing with OD using this command.
5. _MAPDIGITIZE to draw with simultaneous Object Data attachment.
This command is used to draw graphic elements defined with _MAPDIGISETUP and define Object Data field values when creating these elements.
Object Data attached to the elements and its fields values are shown by Map 3D in AutoCAD Properties palette (_PROPERTIES) after the graphic elements are selected. This palette allows editing these fields values – similarly to other properties shown in the Properties palette.
This helps to see presence/absence of OD attached to the graphic elements as well as to browse, input and edit OD field values. However, these capabilities are available only for preliminarily selected file elements. Besides, when selecting two or more elements with different OD, the information about OD is not displayed in the Properties palette. Also, OD is not used in Quick Select (_.QSELECT).