The taper is identical. BT and Cat V will physically fit in the same spindle. However, BT uses a metric threaded retention knob and Cat V uses an inch thread. Also, the flange parts of the toolholders are slightly different so they typically are not interchangeable in the tool carousel/magazine.
So on a machine equipped for BT40, I could manually load a Cat V 40 toolholder into the spindle, but I could not run the automatic tool change.