NIFC is the National Interagency Fire Center in Boise, Idaho that controls resouces for all fire agencies including BLM, USFS, state and also NPS, FWS, BIA, etc. Just like shovels, and water pumps, radio frequencies and equipment are just another commodity to be issued.
Sometime around the turn of the century (millineum) they rolled the old BLM tacticals and commands and the USFS tacticals and commands into one list. Some of the BLM frequencies disappeared, and other frequencies appeared that were not there before.
Additionally, the USFS originally only had 3 tacticals, numbered 1, 2, and 3. In California they gave us 3 more many years ago because of all the large incidents but they are only to be used in California. They used to be called tac 4, 5, and 6.
With this new system, there is a NIFC tac 4, 5, and 6 as well so the numbers conflict. So the California tacs have become "region 5 tac" 4, 5, and 6 overlapping the NIFC tac 4, 5, and 6.
In the late 80's I was on some large wildland fires on National Forest land which used the so called BLM tacs and commands. Even when they were listed as "BLM" and "USFS" seperately, they were used whenever there were enough close by incidents to require more frequencies and I did not witness them being used exclusively by one or the other agency. When I was on the Yellowstone Fires in 1988 we were using frequencies I've not seen listed or used since then. The magnitude of the event as shown by the 9600 firefighters in the Park required the use of a lot of frequencies and temporary clearances must have been obtained the use of them.
By the way "tact" is defined as a keen sense of what to do or day in order to maintain good relations with others or avoid offense. "Tac" is the abbreviation for tactical. I've not seen an agency publication that abbreviates tactical as "tact". Did I say that with enough "tact" to get the idea across?
Even in 2003 on the Cedar Fire the commands had been merged but the tacs had not yet and we ended up on one of the BLM tac freqs even though the fire was a joint operation between the CNF and MVU. But it is true that they can apply for and get emergency new freq assignments especially in a non-populated area like Jellystone, however with the addition of the extra NIFC freqs, plus the Vtac freqs that they have been using.