Okay, I get that there are tutorials on how to set up packet handlers. What I am having trouble wrapping my head around is what information does a mod need to send from client to server and vice versa? Versus what is auto-magically handled by Forge & Vanilla MC parent classes.
How do you know what you need to set up packet handlers for?
Where does that information come from and when?
Is it related to events? Which events? For what? When?
I'm really lost when it comes to figuring out what needs to be done to make sure a mod works in both SSP and SMP modes. Simple explanations and links to open-source/github/etc code that does what you are talking about would be appreciated.