@firstname.lastname@example.org hey! It's not documented yet because it's an "experiment": it sends a
Read activity to your followers. It's only supported by Honk and microblogpub, which will process the reply. Without this "ack", only followers that both follow you and the reply actor would have received it.
@email@example.com a bit, it's intended for sharing a reply to your followers when you don't reply (with no reply and without a ack, they would never be aware of it). The reply actor gets no notification for it.