I see.
As I understand, the ZFX is needed only if you plan on adding another coat after the one you're working on. So if at the beginning of the detail, you intend to apply 3 coats of sealant, you would put ZFX in the first 2, but it is not needed for the third, because the third is the last, with no additional coats to follow.
Now if you're planning on applying 3 coats of Z2, and you already mixed the Z2/ZFX, then go ahead and use it for all 3 coats. But for my plan, I'll mix the Z5 with ZFX, because I plan on adding a 2nd coat of sealant, the Z2. But then the Z2 will go on alone - no ZFX required.