When I make a program, I have the option to make it either "run" or "base".
https://src.mrhitech.repl.co/calcmanual.pdf (backup; the original one seems to have been taken down) mentions a "base" program several times on pages 213-214, but never says what it entails (other than having a big "B" next to the filename).
Whatever I do to the program, it seems completely indistinguishable from a "run" program–a program that runs when you select it and hit F1 or EXE.
I'm using a CASIO fx-9860GII if that matters.
EDIT: NEW INFO:
It seems that a "base" calculator program has different commands available than a "run" program–when you edit the program you are much more restricted–you can't use if, while, or for loops, as well as advanced calculator tools–but you also have the ability to convert numbers between different bases.
That still doesn't answer what you are supposed to use it for, but I hope it helps people answer.