How to create a subtractive or difference morph...

I use PML for this... you could use GoZ or ADPs morph exporter/importer to do the same thing. Assuming you don't have any of these here's how to do it within Poser manually. It's pretty simple.

I'll use my eye socket morph as an example.

Ok so I have the morph that I created for the eye socket and I want to create a blink for the eye socket. This poses a problem because if I build my blink by using the eye socket morph active (the only way you can really do it) the eye socket morph will be baked into the blink morph that I created over top of it. The result being that when I dial the eye socket to 1 and then dial my blink to 1 the eye socket morph, which is baked into the blink, will double up and cause a horrible result not at all like what I intended. Morph telescoping.

The solution to this is to build the blink morph with the socket morph dialed to 1 (obviously)

Load or dial in your new blink morph to 1, which results in the telescoping

Dial the socket morph to -1

Looks like crap right? Not at all the right result.

Export that result as a wavefront object. So socket at -1, blink at 1, export. Leave everything unchecked in the export options. Make sure that only those 2 morphs are active -1/1 and the figure is completely 0.

If you want it as a FBM dial check include existing groups in polygon groups.

Name the morph blink

Export.

Reimport as morph target via target actors properties or via figure menu: load full body morph.

0 out the original blink morph you created, or just delete it. You won't need it.

With the socket morph at 1 dial the new blink morph to 1. A working blink.

You subtracted the baked in socket, baked out the blink, and reapplied it.

A Subtractive or difference morph. The uses for this are endless.

I suppose you could skip the whole export part and just spawn a new morph but I always use the export method. Plus it reminds me to tidy up and keep my naming conventions orderly. There might even be a function in the morph brush for this. I don't know, I never ever use it. I use Blender, Mudbox, or Zbrush for morphing. I don't think I've even touched the morph brush in over 5 years, probably way more.