Confused by user_has_cap hook.

In the documentation here, each of the examples calls the user_has_cap like this…

add_filter( 'user_has_cap', 'give_permissions', 0, 3 );

or

add_filter( 'user_has_cap', 'author_cap_filter', 10, 3 );

I understand that “user_has_cap” is the hook. I get that “give_permissions” and “author_cap_filter” are the functions being called. What I can’t figure is the last two arguments. Neither callback function seems to use them. Why are they there? What are they doing?

Thanks,

Andrew