In admin/marctagstructure.pl you can define an authorized value for the tag. The manual says it does not work, but it is useful? Datafields are made of subfields, it does not make sense to link them to an authorized value. Maybe this feature should be removed? Or at least restricted to control fields?
The AV for the field isn't for content, it's for the indicators, thus the hint (at least in the English localization) of "(if you select a value here, the indicators will be limited to the authorized value list)". Sure, it doesn't work, and if it did it would pretty much never be useful since you would need a field where both indicators had the same valid values, but at least it makes slightly more sense than being an AV to somehow fill multiple subfields. Slightly. *** This bug has been marked as a duplicate of bug 2280 ***
If it is for indicators there is no need of that "feature"...