When designing how drag and drop works in an application, you must understand how Motif figures out what operation gets done when the source and destination of a drag and drop don't match.
For each drag source, an application advertises which drag-and-drop operations are possible and on what destinations it can be dropped. For each drag destination, the application advertises the possible sources and the types of operations. If a source and its destination have two or more operations in common, Motif follows a specific order to determine which operation to use. That order is move, copy, link. The application cannot change the operation that is accepted based on the type of thing being dragged.
For example, application A might advertise that an element can be moved or copied. Application B advertises that the destination accepts copy or link. The intersection in this example is copy. If the destination in application B accepts move or copy, then the source is moved because the move operation comes first in the operation order.
In this example, the user could override the move operation by holding down a modifier key, for example the Control key to make the operation a copy. This will work if the copy operation is in the common set of operations. If the copy operation is not in the common set, then the drag becomes an invalid drag.
The only time matching operations may be a consideration is when you have a destination that could accept moves but prefers copies. In that case, the destination is better off only accepting copies.
It is wise to always accept copy. Accepting copy broadens the scope of acceptable drops. In most cases where a move is accepted, a copy would work just as well. Remember, move is implemented as a copy followed by a delete.