The only valid parameter to this callback is a local or global GUI variable index.
localguivars are expressed as local:#, where # is the index for the variable.
globalguivars are expressed as global:#, where # is the index for the variable.
This callback takes the engine-level data in a UIObject and stores it in a global or local gui variable. The engine-level data will vary from UIObject to UIObject. It is also impossible to specify which data item you want to set the GUI variable to, if the UIObject has multiple items of data.
This is a really old callback and is of limited use to custom GUIs at this time. My intent is to write a new callback that accomplishes what this callback does, but that takes more parameters in order to more accurately specify what data you are interested in from the UIObject.