With my Rpi 3B+, I got a heatsink kit, RS Stock No. 867-9039. Two large heatsinks and one small, and a short length of kind of tape stuff, with the 3M logo and what looks like the word "Adhesive" printed on it. As far as I can see, the SOC takes a large one, and the Ethernet chip a small one. Not sure what the other large one is for.
What I can't work out is how to use the tape. It seems to be layered, and maybe I have to peel it somehow (seems tricky to do that)? But what then? I guess I must cut it up to make little patches. Must I wet it or what?