Another vanilla Cairo question, what are the sorts of references

Another vanilla Cairo question, what are the sorts of references, i.e. ids that can be modified by hints? relatively important to work against an evil prover

presumably not locals or tempvars, but say, implicit references, or the two(?) kinds of let statements (expressions and references)

maybe if there's some documentation i don't know about re: all the id types, relocatable values and so on,

also maybe, is there a pattern that can be used for function pointers?