ACL Inheritance inherit_only setting

the chmod man page says this about the inherit_only setting:

inherit_only

                   The ACE does not apply for permissions, but will apply when

                   inherited.

I don't understand what that means.  Thanks!

In general, I'd love to see a detailed explanation of each ACL permission and each inheritance setting, but so far I haven't been able to find that.  So far the best thing I've found is this: https://www.youtube.com/watch?v=DvohLS_v2_o. I suspect this guy could answer all questions about ACLs, but he only goes to a certain depth of explanation.

Thanks!

Sarah