What are Physics Shape Custom Tags for?

I was hoping to leverage them in CollisionEvent. That would be useful.

 

These are a small payload that travels along with the materials though the simulation step pipeline, and is included in modifier and events so that you can quickly read them. Because they are likely in cache already this is more optimal than looking up data elsewhere (e.g. from another component).
They are small (8 bits) because we want to keep memory use minimal, so they are really only practical for tagging objects rather than storing any values. So for example you might choose to use bit 0 to tag everything that is "soft" and then use a modifier to apply some custom logic to any contacts that have that tag set.

 

Thanks Rory