Can someone please explain what all the differences between a local and a global phantom are?

Here’s what I think I know so far:

A local phantom is a product that would ordinarily be built and stocked but is also an assembly in a higher-level product. The “x” designated that it is only a phantom in that higher-level parts BOM; however, because it has a router when the costs are rolled up you will get a favorable variance in cost. The system will essentially double count the local phantom because the router for the higher-level assembly would have time for assembling the phantom. Also, it doesn’t have the use-up rule that global phantoms do

A global phantom is an assembly that is built to become part of another assembly but never stocked; however, it is possible to stock it. When the cost is being rolled up the system ignores the phantom’s router (if it has one), therefore to accurately cost the product you take what would be the router for the phantom should be added to the router for the higher-level assembly.

Another question that’s been bothering me is “does it make sense for a product to be both a global and local phantom?” They seem like they should be mutually exclusive options, however the person who trained me taught me that “if there’s a yes there, there should also be an x and vice-versa.” According to him his trainer taught him that too.