[CX111]

Ok. Humans have mechanical parts now... but just how much "machine" are they? I'm guessing arms/legs for sure, but what else? It'd be surprising if they were completely robotic aside their brains...

[Princess Luna]

I believe this is somewhat explained in the secret ending to ZXA, but I might be wrong.

[Terminus]

When you play as the 'human' character, and taking the approach that no human ever dies on screen, that they are mostly Machine. Not just bionic appendages or organs but a whole swap. The degree of which and how (frozen beef theory, anyone) is the real question. Are they brains in a machine body (cyborg to an extent)? Is their minds copied to a body that looks just like them (ala Weil)?

[Oakie620]

It is repeatedly stated in ZX Advent that humans receive bionic bodies (keyword: "bodies"). The Sage Trinity themselves have lived centuries due to their mechanical bodies, and Vent, Aile and Ashe have all shrugged off physical trauma that could be otherwise lethal to unaltered humans (they also explode in the same manner robots do when they die, might I add). However, the wording from one of Albert's ciphers (code UC80) implies that not all humans receive bionic bodies ("All Humans who get a bionic body have to come to Legion for inspection. I made sure that every bionic body part I inspected was infused with my DNA.").

I've been told that ZX Tunes elaborates on this, revealing that humans may be partly or fully modified at a certain age and given a "Hu" number, though I currently lack the means to recheck this statement, so take it with a grain of salt.

The subject shouldn't be surprising though since the theme isn't unprecedented: this is pretty commonplace in the Legends series, where many Carbons resort to bionic implants (even Volnutt swapped most of his body for mechanical parts). If anything, it further cements ZX as a significant stepping stone in the transition to Legends.

On a side note, an unrelated series that wonderfully explores the implications of bionic enhancements is Gunnm (a.k.a. Battle Angel Alita) by Yukito Kishiro.