I give you a Example.
Imagine you have a Inventory on your Character filled with Items. You create a Widget localy to display them and interact. If you click a Item you notify your Character what you clicked and your Character in turn can tell the Server what he clicked. The Server was not involved in any UI buisness.
If you think of something like a Trade window between two Players you have a local UI for “each” Player and not a single one both interact with. So P1 Drags a Item into Trade via his local UI, P1 Character would tell the Server that he added ItemX to trade, Server would forward it to P2, when P2 recives the message he tells his local UI to display ItemX. Then both hit the confirm Button that again will go trough their Characters to the Server who in turn makes the actual exchange of Trade goods and send a message to both Players afterwards so they know when they can remove their no longer needed local UI.
Hope you can See on this Example that the Server never knew or interacted with the UI or the Trading Players never tried to access the others UI.
Good luck and have Fun =)