Admin Center: Enforce PWd Policy at a group level: How does it work?

Documenting the Groups Management Module.

I do not understand how the "Enforce Pasw Policy" available as part of the "Manage Groups" module works.

If I enforce the Policy at a site level (so as I understand it this means that all users will have to comply with such a policy, I can (re)enfor4ce it a group level. What does it mean? If I want all users to comply with this policy why could I say that the group Users do not respect this rule? What if a user belongs to one group which does not enforce the password policy and to another group which enforce it?

If I turn off password policy at a site level, this option disappear in the Group Management interface. Should'nt be the opposite? aka if you do not enforce a pwd policy at a site level a webmaster could still decide to enforce it for a dedicated group only (e.g: external customers group only)? This would make more sense to me....