Appearance. I'm sure Masamune1600 or someone will explain in greater detail. But that's what Jenova does. Almost all sightings of Sephiroth in FF7 are Jenova taking on his form. Jenova just made Kadaj take on his form. Though the way Masamune explains it, it sounds like it's the same either way as Jenova and Sephiroth have like this mother/son telepathic link thing where their wills are basically the same.