Maybe the video of my presentation at MODXpo will help.
I think one problem a lot of people have with MODX permissions is thinking they're more complicated than they really are. I know that was true for me for the longest time. Another big problem is the carryover for former Evolution users, especially with respect to Roles. That bit me too.
My best first bit of advice to people trying to figure out Revo permissions is to forget about Roles altogether. Having a bunch of Roles is confusing and usually unnecessary unless you're doing, say, a University or corporate web site. In Revolution, you can almost always do what you want with just two Roles: Admin Super User (for you only) and one other role.
The only reason to have more Roles is if you want users in the same User Group to have different permissions (i.e., different Policies apply to them). As long as you put users with different rights in different User Groups, and all the users in a given User Group have the same powers, roles are irrelevant.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html