Pretty much what it says on the tin.
I don’t see anything in the index, and I am quite possibly the worst at reading PDFs, so if there is something, there’s a damn good chance I’ve missed it.
If you start the game with a mentor then that skill is their profession.
Does it do . . . Anything else?
Just what it says in the “What’s your Specialty?” heading. Start at 2 or up it to 3 or 4. Kind of gives you a role in the group, as well.
So the only reasons to underline it are to make it always clear for prohibition of other PCs with that Specialty** and to note that Skill that your beginning Mentor has?
** The specific wording seems to proscribe even pre-existing PCs with a matching Specialty from another context being brought into play.
If another character is later brought into play, it cannot have the same specialty as any other character in play.