exclusively used by a defense to match personnel. So if Christian McCaffrey runs out he is considered a RB. If Travis Kelce runs out he is considered a TE.

Now they may line up with either of those guys split wide, but in general a team would still call it based on the position they are in most of the time.

So for example you have a team with both of those guys and another TE. It would be called 12 personnel and doesn't change if both are lined up as WRs.

It is actually what makes both of those guys so valuable. Normally a TE or RB means a LB to match up to stop the run. But with both of those guys they will beat most LBers in the pass game.

On the flip side it is also what makes Fred Warner so valuable. He can stay in the game regardless of personnel as he is great against both the run and the pass.