It's really pretty simple, actually.  If I purchase a product that depends on the Morphs++ package, and/or the Elite morphs package, I know I have to inject those morphs BEFORE I inject your morph for it to work properly.

So, if you're the seller of the morph, you must list Morphs++ and/or Elite morphs as a requirement for your product.

You can create your morph on a figure with those morphs injected, but you cannot distribute those morphs - only list them as a requirement.

Here's a good example product - note the required products:

http://www.renderosity.com/mod/bcs/vh-ariena/94669