I've been searching the documentation, but have been unable to find anywhere that describes what the various permissions available for a shared folder mean.

I know that 'r' equates to setting "Viewer" from the GUI and that 'rwidx' equates to setting "Admin" from the GUI, but what do those letters actually -do-?

I have some folders I would like to share such that certain users can read messages and mark them as read, but not delete or modify messages in the folder... is there some combination of 'rwidx' that would enable this? Using just 'r' prevents messages from being marked as read in the users' MUAs.

I suspect, but am not sure, that 'd' allows you to create subfolders.