It’s a “dibs queue” that only locks you into the Bruiser role for the initial hero select screen.
After that you can switch to DPS, then back to offtank.
However between the 3 DPS, if one of them picks offtank, then it prevents the other 2 DPS from picking offtank.
So if you swap away from offtank, and somebody else grabs an offtank, then you’d be blocked from going offtank.
It’s purely in there to prevent squabbling if you get two offtank mains on the same team.
Additionally, if the matchmaker can’t find an easy bruiser-preferred player, then it just grabs 3x DPS.
So it’s not exactly 3 queues for the matchmaker, but it’s pretty darn close to it.
I kinda assumed you read the concept. Or at least the first 9 bullet points.