I've just uploaded to new-file-storage a document called "ACS 4
Permissions Tediously Explained". As the name clearly states, it's a
document that will help you understand how the permissions system in
(Open)ACS 4 works. Great read.
It's at https://openacs.org/new-file-storage/download/permissions-notes-vadim.html?version_id=229.
The document was sent to me by Vadim, an aD developer, as a response
to a question I posted to this bboard a few weeks ago. I published it
with his permission.