Discussion

ok thanks  

it was once intended for something else what do I have to change that I can use it

The IMessage interface is entirely unnecessary. Other than that, yes, that is how you add packets.

yes I have this variable (it goes from 0 to 109).   this is what i'm trying right now. Edit: it works like this, but how do I find out when I have to move a slot outside of the visible area if I shouldn't use the slot's position   I've "tried" something like this before in another mod. But I am not sure whether it works that way and whether it is even right? code: PacketHandler: https://github.com/Luis-st/Forge-1.16.5-36.0.1-mdk/blob/main/forge-1.16.5-36.0.1-mdk/src/main/java/net/luis/cave/core/ModPacketHandler.java my message interface: https://github.com/Luis-st/Forge-1.16.5-36.0.1-mdk/blob/main/forge-1.16.5-36.0.1-mdk/src/main/java/net/luis/cave/core/IMessage.java SyncSlotPosition message (not final/ in work): https://github.com/Luis-st/Forge-1.16.5-36.0.1-mdk/blob/main/forge-1.16.5-36.0.1-mdk/src/main/java/net/luis/cave/core/SyncSlotPosition.java   is there an error somewhere / does it work like that way?  

Sorry i didn't know thought it was the same thing. Edit: forgot to say i changed account because the other one wasn't letting me in.