A friend and I started playing Starbound together yesterday and, this morning, I had a message from him saying "yo I'm dling some mods, pretty sure both of us will need them installed so I'm gonna send them your way too". The thing is, we have really different taste in mods... I really don't need a Power Rangers suit mod in my game. Also, he wants to install the Remove All Particles mod to improve his FPS and mine are just fine. Do we both really need to have the same mods installed in order for him to have access to his mods while we are in Multiplayer mode? Is there a way to know if the mod even works in Multiplayer?
If it is a change to existing art then you do not need it. If it is new content such as the power ranger armor then you need it in order for it to show up on both ends. Changes to scripts on objects and items need to be server side, but changes to for instance techs do not necessarily. Basically, if it is something that pertains only to the player and not the world, it is client side. If it changes a script other than a tech or a config other than for the player, it is server side. So you could have a food doesn't rot mod and it would not rot in your inventory; however putting it in a container it would be subject to the same rules. Same with stacking. If your server doesn't have food stacking for example but you do, then the server's containers would reduce the stack size to 1 (eating your items!).