Wl_data_device_manager.Dnd_actionDrag and drop actions.
This is a bitmask of the available/preferred actions in a drag-and-drop operation.
In the compositor, the selected action is a result of matching the actions offered by the source and destination sides. "action" events with a "none" action will be sent to both source and destination if there is no match. All further checks will effectively happen on (source actions ∩ destination actions).
In addition, compositors may also pick different actions in reaction to key modifiers being pressed. One common design that is used in major toolkits (and the behavior recommended for compositors) is:
Behavior beyond that is considered implementation-dependent. Compositors may for example bind other modifiers (like Alt/Meta) or drags initiated with other buttons than BTN_LEFT to specific actions (e.g. "ask").