Emissions mean light. You can set up any object to emit light with either that ambient plug in firefly or the emissions node in superfly. The closest thing to masks in the real world is probably stencils you would use with painting. You would use them with a blender or mix node to tell poser to put one kind of shader in one portion of an area while using another in a different portion. Here is a sample with a mixclosure node. the compound nodes are just the 2 shaders compressed to make it easy to see the set up for the mask. the image mask node is just an image node renamed.

Here is a sample with the blender node. You can use it with the poser surface root node also and plug it in anywhere you want to combine 2 shaders or even colors. I'd give an example, but Poser decided to stop working and I'm waiting for it to crash so I can restart it.

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