I was confused by this too, so I asked :)

I think it largely depends on the gallery.

For example with Photography if you add 3D elements and / or heavily post work the image, then that's considered mixed medium.

For Poser, apparently so long as the image contains a Poser figure in it, it can remain in the Poser gallery even if heavily post worked, provided it isn't an advertisement for a product, in which case it would belong in the Product Showcase gallery.  However, for me personally, I feel that if an image rendered in Poser has been really heavily post worked, then I think those should be in the 2D or Mixed Medium gallery.

Something like Collage art could be either 2D or Mixed Medium IMHO.

Scanned images such as the pattern of your granny's quilt that you added a frame to in Photoshop I think would be considered "Mixed Medium".

It's not really a cut and dry type criteria.  If we aren't sure of something when we go through the gallery, we ask for the views of other staff as to whether it should be in the gallery it was posted, or moved to another one.

The number of programs you use to create your image is not a factor as to whether your image is ""Mixed Medium", because if that were the case most every image would need to be posted in Mixed Medium because not many just use one program IE: Poser to create their art. Some set up in Poser and render in Vue and then move to Photoshop etc. So 1 program or 10, it doesn't matter.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi