What is a type "x" message?

What is this message in the streaming market data? I don’t see type “x” (“T”:“x”) described in the API docs.

[{“T”:“x”,“S”:“UPS”,“i”:79371806259278,“x”:“D”,“p”:214.655,“s”:10,“a”:“C”,“z”:“A”,“t”:“2021-12-28T16:14:49.427742253Z”}]