XIM is a frontend that provides communication function with X Application. When you're using Gtk/Qt application, you should use im module instead of XIM.
There is only one configuration entry, which will use On The Spot style for XIM. This might cause the input window cannot follow the cursor in some application which works well when using Over The Spot style.
Opera need this option to be enabled to make input window displayed at the cursor position, but still not perfect.