Hi,
What is Particle System Primitive?
How useful about it?
What ‘s different with point group?
I can’t find some document.
Thanks!

The particle system primitive was used by the old POPs system where you can have a single primitive that had the particles as its vertices. Don't use it

100% with Ed. No need for old school users too to use the Particle primitive either.

It is far more efficient to deal with raw points now. No primitive attribute overhead. Everything that was edge cased to particle primitives including auto-instancing spheres/tubes/sprites/etc. are now easily supported on raw points.

Don't use the particle primitive.

I thought there is one reason to use particle primitives and that was to show points as guides for an HDA. Is there an alternative for this?

With HDA's, we added the Guide option to point at any SOP/Object inside your asset including raw points which should work as guides.

Does that work in HDA's?

You are right Jeff, it does work. I thought you needed a particle system to make sure points show up as guides. I just tried and having plain points in the guide gives you a lot more flexibility than a particle system ironically as the Display Options for particles allow you to customize plain points this way more so than a particle system.