The "Stop" operation stops the transfer only if the transfer is in state "Suspended" or "Transferring".

However, there is a definition hole in conjunction with "EndNoLaterThan" and more than one attempt to transfer the data:

The status "transferring" is defined as having bytes moving on the wire. That means that the state of the DTI must be a 
different one when a transfer attempt failed (but the maxAttempts requiremet has not been reached) and the next one is 
in the process of being set up. In that phase, the transfer is not stoppable as it is not (yet) in state "Transferring" 
or "Suspended". 

I prpose to expand the notion of stopping a transfer regardless in which state it currently is. The effect of the stop 
operation will be nil if the state is either "Done", "Failed" or one of the substates of "Failed".