X Macros! Mostly because it's one of the more understandable and funky things you can do with the preprocessor. You can do some crazy stuff with them :)
Oh, that has a name. In C, often used with an include file instead of a body macro, and often (?) used where code wants multiple internal representations of some table of data.
https://blog.gboards.ca/2020/02/adventures-in-obscure-c-feat...
https://en.m.wikipedia.org/wiki/X_Macro