fusing geo using uv attribute (eg. separated head from neck)

Lets say you've separated the head geo from the body for rigging purposes or otherwise.
How do you glue it back together reliably, without resorting to many individual vertex pair selections and fusing each?
UV's! As long as your mesh has good non overlapping UV's (which existed before you chopped it), you can promote them from vertex to point (might as well give it a new name, and keep the originals for actual texture use) then specify this new attribute at the Match Attribute (see below). Turn off Snap Distance, as the UV data is what will snap vertices/points together (in this case I've called it fuse_uv). I've left everything else in the Fuse node as default.
This is super handy especially if your model's point count is not fixed (eg you're not sure where you're cutting the model yet)

attaching nulls to parts of a moving/deforming object. rivet.

At SOP level, select 3 points of the geo you need to attach a rivet to, this is to calculate rotation (if your object/mesh-area is rotating/deforming)

Make a group with these points, call it something like "pivot_group".

 Go up to Object Level and make a Rivet node.
Here, specify the SOP eg - /obj/geo1/object_merge1

and the pivot_group in the Point Group dialog box.

You can now parent a NULL or something else to the rivet. Something I noticed, was the rotation of the null can be a bit offset, and I've been manually adjusting the rotation...

If you're exporting a null for Flame, you might be better off parenting a camera object, as the alembic export seems to ignore the null. Maya will import a null, although it will show up as an invisible point, so again you're probably better off using a camera for this too.

I've been exporting the null using the File-Export-Alembic tool..