Hi Karla, and an excellent question.
Basically, XXZ and XYZ powers are powers that require an X and Y gene to express, but they also need a Z. A Z, as I'm sure you know, is a gene or sequence of alleles expressed anywhere other than the sex-linked traits. So an XXZ or an XYZ power is one that requires all of X, Y, and Z in order to express itself as one coherent power. Sometimes it's an XX/XY power that's closely related to the subsequent Z power, such that it effectively blends into one power. Other times, it's an XX/XY or Z power that needs an Z or XX/XY to trigger or activate.
In short, though, it's a power that requires all three types in order to be expressed.
They aren't on the powers list because, for the most part, they're limited to canon only. We may allow very creative inventions for XYZ or XXZ powers from members, but you should ask us first in the questions forum. Most of the time, these powers are reserved for elite characters, such as hegemon higher-ups, and as a result, most characters with XXZ/XYZ are 2 asterisk characters.
Also, XXZ and XYZ powers are supposed to be fairly rare, even amongst the powered environment, and because of the sheer amount of variability when so many genetic factors are considered, they're each 'unique.' Hence it's counterproductive to place them on the powers' list, as (1) they are not readily available, and (2) rarely would two XXZ/XYZ powers be the same.
I hope this helped. Thanks for stopping by.