Means nothing, and should not be used.
Copy the data.
Move the data, i.e. first copy it, then delete it from the source using the DELETE target of the X selection protocol.
Add a link to the data. Note that this is only useful if source and destination agree on what it means.
Special action which tells the source that the destination will do something that the source doesn’t understand.
Ask the user what to do with the data.