Yep exactly. It deals with activities from Mastodon, Misskey, etc etc and tries to bring order to chaos. Lately a lot of the code has been moved into "validator" modules instead where it forces objects of each type to conform to a schema. Eg here's the one for ChatMessage: https://gitlab.com/soapbox-pub/rebased/-/blob/develop/lib/pleroma/web/activity_pub/object_validators/chat_message_validator.ex