What's the logic behind Attache?

I’m curious about the in-fiction and/or game design logic behind allowing Medics to come in as a third specialist on missions, and why the ability is called “Attache.”

From what I can tell, the usual implications of that word are either or both

1: a expert advisor or assistant attached to a higher-ranking official as a permanent member of their retinue or

2: a member of one organization loaned out to another organization to provide specialized skills or knowledge.

The first does not seem to apply here; Legion Medics are just as free-floating and autonomous as other Specialists, able to lead squads and not paired with specific officers. The second would be a perfectly reasonable way to set up the fiction for medics in a mil-fantasy game. We could have treated Medics like Mercies or Alchemists, portraying them as members of a religious order who travel with the Legion but are not members, in which case it would be very logical that they wouldn’t count against the traditional Legion deployment. But in fact it seems that Legion medics are oathsworn soldiers, not embedded civilians.

The best I can come up with is that the in-game commanders and/or the game designers felt that a successful squad needed a certain amount of 2-threat offensive fire power and that it was too hard to justify giving a Specialist slot to someone who did not provide it.