Thing "uniqueness" attribute undocumented
 

I'm seeing quite a few references to the attribute "uniqueness" on things, but this does not appear to be documented.

Can someone shed some light on what this does, and what are the options?

Whilst on the topic of uniqueness, I'd assumed that the "isunique" attribute determined uniqueness within a container using the compset. In other words, you can only have one "Alignment". However, I also see that "Skills" are also unique, but obviously you can have multiple skills in the container. Is there some other way to limit things like "Alignment"?

Mathias may correct me in this, but uniqueness is generally one particular instance within a set on the character. I'm not certain what the difference is between the "unique" and "useronce" options. Presumably "none" works like "isunique=false".