Are they using payloads to loft bounding box info and unloading payloads to control visibility?

If not, are they even following any USD conventions or is it a Houdini-specific convenience thing?

The original payload controls still exist as-is, and the expansion states docs make no mention of payloads, so any clarification on this would be very welcome, thanks.

Have you read https://www.sidefx.com/docs/houdini21.0/solaris/support/expansionstates.html? [www.sidefx.com] If that documentation leaves particular aspects unexplained, please let us know.

But to briefly answer your question, Houdini only sets standard USD "draw mode" or "visibility" attributes on prims in response to expanding or collapsing prims in the tree.

The expansion state doesn't not interact with payload loading, because payload loading control is a lot less flexible, requires that your assets have payloads set up, and provides very little benefit for this use case over simply setting the draw mode attributes.

Hope that helps!

Thank you for the clarification. I did read the docs, but they didn't explicitly mention that it wasn't payload-related, and I'll confess to not being aware of the Draw Mode functionality in previous versions. I previously believed the only way to get a bounding-box drawn on your prim was via a payload. The more you know...

I'd like to add that if the docs specifically made a mention about Draw Modes, that would probably have set me on the right track.
Yes, the Expansion State menu does have "Bounding Box Draw Mode" listed, but the Draw Mode column has always been hidden by default in Solaris and for users not aware that "Draw Mode" is a thing, you don't draw a connection.

Thanks, I'll mention that to our doc team!