Membership plugin - explanation of rules logic?

Sorry, I'm new here so this maybe just a matter of not yet know my way around.

I am looking for an explanation of the logic behind the access rules in the Membership plugin. Is there a manual I cannot find?

For example, if I want to include some BuddyPress groups for a membership level, is it better to to allow the groups I want them to access, or disallow the groups I don't want them to access? Is there a difference? What is easiest to maintain if I keep adding new groups? Which rule wins if two rules conflict? Can I create a rule that applies to all groups rather than specific groups? If I leave groups out of the allow and disallow rules, what happens? That sort of stuff.

I could spend time setting up dummy groups and levels and go with trial and error to work it out, but I'd rather not rely on trial and error if there is a proper explanation somewhere.

Thanks for any help in pointing me in the right direction.