In Igros Moshe, the classic compilation of responsa by R' Moshe Feinstein, each of the published volumes contains one or more collections of numbered responsa, each concerning a specific area of Jewish law (generally, one of the four turim of Shulchan Aruch, but there is also at least one collection on Kodshim and Tahorot, which don't get their own section in Shulchan Aruch). So, for example, Volume 1 contains Orach Chayim - Section 1 and Kodshim Vetahorot - Section 1, and Volume 5 contains Yoreh Dey'a - Section 2, Orach Chayim - Section 3, and Even Ha'ezer - Section 2.
This layout presents an annoyance for anyone who hasn't yet memorized which sections are in which volumes and wants to look up a particular responsum based on a reference. They have to guess which volume the referenced section is in, flip through that volume to see if it's there, and then try again if incorrect the first time.
So, as an aid to the researcher, let's make a quick-reference list of which sections are in which volumes, including, where available, links to the beginnings of the sections on HebrewBooks (or any other online repository that has them). Then, someone following a reference could go straight to the right volume.