Nitreug,
Poser stores geometry info in the pz3 so that it doesn't screw up your original files. Lets say you loaded Victoria and used the grouping tool to add some new materials...for example, made all the faces on her right hand a new material called 'glove.' In the obj file loaded in your memory it adds a line 'mtl glove' before all the 'f' lines (faces) that make up the hand. Now you want to save it to a .pz3 (or a .cr2 for that matter, honestly) it can't re-write Victoria's original .obj file because then Victoria would load with a glove material from whatever .cr2 or .pz3 you loaded her from. Instead, Poser creates a 'customGeom' inside the .pz3 or .cr2 for any body part you used the grouping tool on and when it loads, instead of calling the original body part from the .obj, it loads the custom geometry that's saved there.

Now, lets say you were making some sort of skintight suit for Vicky (or any other copyrighted character) and used the grouping tool to assign a new materials all over her body. You save the .pz3 or .cr2 and put it on the net for people to download or buy. Anyone with a text editor and a knowledge of the .obj format can extract the info they need from the .pz3 or .cr2, save it as an .obj and open the file in any 3d program that can import an .obj. Whether they own the original Vicky character or not, whether they own Poser or not.

I hope I'm making things clearer and not confusing the issue more.