Sequencial messages

I know that Openfire sends messages to the clients sequentially. So if the client A sends x, y, z, client B will receive x, y, z and not y, z, x.

I read this specification XEP-0198: Stream Management and it says: “The sender does not need to wait for an ack to continue sending stanzas”.

If so, how Openfire can provide sequentiality in sending messages?

In the example before, if “y” has not been acknoledged, how Openfire can send “z” and mantain the sequentiality?