A relay could have a simple algorithm for what it decides to share. Or, a relay could have a complex algorithm for it.
For example, a relay could share anything shared with it. Or, a relay might share everything from a list of users. Or, a relay might share posts with the word "BANANA" in it. Etc etc etc.
But, from the point of view of a server listening to a relay, it isn't aware of what algorithm the relay is using.
...
