What is the need for receive_sender policy rules?

Hi all!
I started developing applications with D-Bus recently, so please bear
with me. The application that I'm developing will connect to the
system bus.

I have seen that even in the simplest policies (such as the one in
org.freedesktop.timedate1) there are two rules defined for every user:
"allow send_destination" and  "allow receive_sender". However I can't
grasp why the second one is needed when we have these rules in
system.conf:
allow receive_type=3D"method_call"
allow receive_type=3D"method_return"
allow receive_type=3D"error"
allow receive_type=3D"signal"

The only reason I can come up to for having the "allow receive_sender"
rule in every policy is so that if system.conf changes, the
application would still have enough permissions to carry on with its
task. Is there anything that would get blocked if you remove the
"allow receive_sender" rule with the current system.conf file?

Thanks for your help!
-- 

Kind Regards.

Sergio A. Ruiz Peña
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