A multicast means an event/function that happens on the server will then be reproduced on all clients.
Replicated is just a variables state, for example in the tutorials I linked above? Lets say the chest has a Boolean value that is replicated. In the function it might for example say when this is opened set the Boolean to true.
Now the event that fires this? Let’s say it is non-replicated. The initial person entering the trigger for the chest to open sees it open as the function got called.
Next person however does not and cannot interact leaving it in a broken state to them. They see it as closed as the Boolean already states it is open and therefore cannot be opened again. But they never saw the chest open.
A multicast would make the chest open for all. While this is good behaviour on paper? If they cant see the chest? The multicast was wasted bandwidth and draw calls.
Some things you might not want to multicast at all and instead just run on server which replicates it back down anyway thus creating twice the traffic for the same result.
The tutorials in my original post explain this much better and simpler and definitely worth a watch before attempting any multiplayer game creation!