The class callbacks is the superclass of all the CAPI objects that receive callback calls in response to user gestures, excluding output panes. This includes collections and choices, buttons, menus, menu components, menu items and item-pinboard-object. The actual interaction depends on the specific class.
The arguments that callbacks are called with can be specified by the initarg :callback-type. When the argument contain the interface, the actual interface can be specified to be another interface by using attach-interface-for-callback. The function element-interface-for-callback can be used to find which interface is going to be used in a callback.
Callbacks can be aborted using abort-callback.
There is more detail about the callbacks available in choices in Callbacks in choices.
Note: output-pane and its subclasses implement callback calls by the input-model mechanism.
CAPI User Guide and Reference Manual (Unix version) - 3 Aug 2017