When Alita gets put into the second body (the URM one) she notices there are jets on the arms, which Doctor Ido explains are plasma jets.
Does she ever actually use these? It seems weird that they had a mention but then didn't get used, but I don't remember ever seeing them being used - am I mistaken and she did use them?
She uses the plasma to activate the monomolecular sword that she gains in the film.
The use of the other plasma outlets isn't shown in this film, but maybe later ones. Cameron has stated the desire to make two more films if the first one proves a commercial success.
It's possible that the jets are there to augment speed/agility in the same way as Doc Ido's axe is accelerated by a plasma type jet.
As you're probably aware, there's a pretty large body of work for this character in terms of print material. There's a lot of technology and backstory to draw upon.
