Contents of plus.j2a are explained in the second part of the ANIM::Set list in the AngelScript readme (Violet why can't I link to specific PLUS_ constants). The animations you brought up are all unused by the game itself but can be loaded by scripts - although usage of the first one is discouraged due to it being a work in progress and subject to change, as mentioned in the readme; the other two were included as commonly occurring tiles that you may find useful to have available without having to edit a tileset, similarly to why texture constants were added, and also because they happen to well correspond to JCS events Fire and Lava, even though, as I said, neither of those events has any implemented logic to load and display them and I don't know of any plans to do so.