Dually fender seal?

I guess it's a seal, I have an 88 Squarebody dually and I'm going to be pulling off both of the rear fenders to fix the the bondo mess on the bed sides. My question is, when I put them back on. What do they have between the fender and the bed, is that a seal of some sort, if so what is it, do i need it. One side of my bed has damage and bondo under the fender and it looks like they just bodo'd the gap between and sprayed the whole thing. Other side is stock and has the thin seal between the two. So tell me what I need to put there. All help appreciated. Thanks!

Come on, I know some of you
Guys know....

it is a seem sealer from the factory.... and not you dont to put it back on there the fenders usually fit pretty tight next to the bad side without it... but warning that shit is a bitch to get off usually lol if you wanna re seam seal it after repairs you can us 3m fast and firm seal sealer