How to know the meaning of subroutine or function

Dear experts,
When I use user routine to achieve my goal, I may call many functions or subroutines, just like
FLABRT(“mgdraw.f”, “Failed region name conversion.”)
GEON2R(“regmixxx”, region_number_out, IERR )
…and so on,
I think it is built-in functions or subroutines, but I have not find anywhere to explain it.
Looking forward to your help.

Dear @xiongbp,

Unfortunately, the documentation is not abundant on this topic. Some material is being prepared for the upcoming FLUKA Advanced course and that will definitely improve the user experience.

In the meantime, you can have a look at the material from the last advanced course, namely the lecture on Fluka environment where on slide 44 you can find something about the mathematical libraries, and more importantly the lecture on the source routine at the slides 31-33.