Help me understand "Action Groups", please
 

OmniFocus users seem to endorse a concept called an Action Group, which appears to be different from a project, subproject, or folder. Is there discussion somewhere of what an Action Group is?

Thank you.

AFAIK, Action Groups exist to allow you to set the task sequencing flag to a setting that is different from the rest of the project. For example, you have a project where tasks defaults to parallel, but there is a portion of the project that needs to be performed sequentially. Action Groups allow you to do this.

Yes, there are lots of discussions here about action groups. Sometimes they use the terminology "parent/child" as well, so you might search for those terms as well. I won't try to summarize the very diverse discussion except to say that some people think the current implementation is very poorly designed, but we don't all agree on what the best way to fix it is.

What yucca said is one way to use Action Groups (and what I do, although I think they are deficient in some important ways in this regard). Some people use them more like subprojects.