So I've been starting to see a lot of questions related to the specialists tab and what that does, so I felt like I'd explain that, starting on O and working to D.
3DHB: These will be your receiving backs, they'll come out of the backfield and be essentially another weapon. Players like this include Marshall Faulk and Alvin Kamara.
PWHB: on certain sets, most likely 2+ TEs and a FB, you're powerback will come in. These are guys like Marshawn Lynch, Earl Campbell, etc. who are tough to bring down.
SLWR: This guy is your slot WR. He lines up between the end of the Offensive line and the outside WR. Generally smaller, but they're the one who's going to break several tackles. Slot WRs that come into mind are KJ Osbourn and Brandon Aiyuk.
SUBLB: for sets that don't have all 3 linebackers on the field by default, you choose who's on the field here. It's very common in Madden for people to put safeties here, as they animate better than linebackers.
RRE + RLE: For a set that doesn't have all 3 linebackers on the field, you can pick who comes off the edge, whether it's your defensive end or one of your outside linebackers.
RDT: Again, in the set where all 3 linebackers aren't on the field, you can pick who's rushing from the interior linemen spot. This can be either a DE or a DT.
SlotCB: In a set with 3 or more CBs on the field, this CB lines up between the linebackers and the outside CB. Usually lines up on the slot WR. Slot WRs usually have good blockshed and hit power. One that comes to mind is Jalen Ramsey, although people places their smaller CBs here, as height doesn't matter as much in the slot.