Serial number details in specification

[Thomas Kluyver]

On Fri, May 10, 2019, at 12:16 PM, David Rheinsberg wrote:
> The bus uses the serial number to track "reply-slots", meaning a
> pending method-call is tracked by its sender+destination+serialnumber
> combination, and this is used to match the corresponding reply.

Out of interest, why does the bus need to match replies to requests? I thought it just sent both method calls and replies to the destination specified in the headers, without matching them up.