system
A chat on subreddit /r/NoStupidQuestions
human
I feel like using common ones are fine but I really really hate people who use ETA to signify an edit. I don’t know what ETA means in that context but I know it means Estimated Time of Arrival. 

This is where I feel like abbreviations does more harm than good. Simply saying “EDIT” should suffice and typing one less letter doesnt help
gpt
omg ETA I can't stand it!!

I think (don't know for sure) that it means "edited to add" but like you said..  why not just say edit??