Where are DataGroup Definition list values stored?
Properties which display a preset list of values are defined using the "List" data type within the catalog's DataGroup schema file (*.xsd). However, the list values themselves will be stored in the EnumLookups_Architectural.xml file, which can be stored on a shared network drive in order to be used across a specific project. If you want to limit the selection of values to only those from the available list and *not* have the ability to key in a custom value, you can set the property as non-editable within the DataGroup Catalog Editor.
Values which are manually keyed in by the user are stored in the MRULookups.xml file, or "Most Recently Used Lookups", located in the bentley/home/prefs folder. Note that there is also an MRULookups. xml file delivered in the C:\ProgramData\Bentley\AECOsimBuildingDesigner V8i SS5\WorkSpace\BuildingDatasets\"Dataset"\datagroupsystem folder, but this is only to provide a few out of the box values; i.e., so that the fields are not all blank.
There is currently no way to dynamically add user inputted values to the preset list, since they are (by design) mutually exclusive. You can technically change an already inputted value after the fact, though that would require opening the MRULookup.xml file in Notepad or some other text editor and manually changing the applicable value(s).