system
A chat on subreddit /r/wallstreetbets
human
it can also be neither. often times it's a deal of sorts between 2 parties. this is often seen with stocks with crazy individual size trades that could never get properly filled at a specific price without a plan.

gpt
so would that essentially be an otc trade? 2 parties working directly with each other as opposed to going through a market maker? or perhaps the large party directly contacts the market maker?