Fixture ID has two related contexts.
One specifies the fixture offset used to model the part within FeatureMILL, especially in multiple fixture situations.
The other context specifies the fixture offset used to produce the part in NC code.
For NC code, FeatureMILL passes the Fixture ID to the Post Processor that then uses the reserved word Fixture to pass the fixture offset information to the machine. While your part may have been displayed and modeled at one location, the fixture offset may override that location in actual production depending on your machine tool system.
You must set the Fixture ID to correspond to your machine tool. If your machine uses G54 or G55, set the fixture ID to 54 or 55. If your machine uses H1, set the fixture ID to 1.
The other fixture offset type reserved words, Datum Shift and Datum Set, are not supported. Datum Shift and Set are commonly seen as G92, or G97 codes.