Modding environment
Minecraft Environment infrastructure has to be considered when using modded worlds. In singleplayer your modpack has to include server only and client&server mods too because the server is on your machine. Minecraft and your Mod loader will handle which mods should go to the client or the server side parts of the infrastructure.
In multiplayer you have to split your mods between client and server otherwise you are adding useless code with resulting bad performance and could also create problems.
client&server Project types has to be included on both ends.