A simSet object is a list of objects. It is similar to the simGroup, except every object is in exactly one simGroup, while it can be in any number of simSets.
If you want better performance, you can use variables on a scriptObject (like %so.brick[%n]) if you add entries and never remove them, and have some mechanism for preventing duplicate entries, but unless you are scanning through and adding 10000s of bricks, a simSet will be far more than enough.