My understanding is if you were to run your own relay all your notes and the notes of other npubs you follow (and probably other criteria you specify) would get pulled and/or pushed into your relay for you and others to use.  I would want to set up a cluster of relays among family and friends to know for sure I can communicate with those people at a minimum.   One thing I haven’t researched or read is if the nostr protocol has a way of knowing if you’ve ever missed any notes/data. 

 Thanks. This is my understanding too.

Not sure gow relays sync the data. There must be some mechanism to make a light lookup if your rely is up to date against the other one based on specified rules.