What are the rules for the use of words that have a variant ending in -al? [duplicate]

I am not sure what relationship you are referring to, but if you mean does the -al ending mean the same thing in all cases, the answer is yes, more or less. However:

• numeric and numerical are actually synonyms or variants

• likewise electric and electrical, more or less

• electoral doesn't just mean "related to electors"; it also means "related to elections"

EDIT: What I would say in the case of numeric and electric is that the -al suffix does not actually add the "pertaining to" meaning in the same way, because the -ic ending already does so. As to why there are both forms,...that's another question.