Most, if not all, ProxSets are somewhat arbitrary. But ProxSets are needed, to focus attention. Yet they also need to be flexible.
You need to use ProxSets to focus attention, but you need to allow them to change. It goes back to the Core Idea of ProxThink: “In a situation, change elements, relationships and the proximity to better relate to each other.” In this case, a ProxSet is an element in the proximity of your situation.