how to create a module like acl?

hi, i'v read all articles about node access, and still have this one question:

when somebody created a node access module, and implements hook_node_access, why should they put their realm in node_access table too (using hook_***_grant)?
and if they do, When will it be checked?

I thought this is the best place to ask since you have already done this.
thanks in advance.