Pretty straight forward really:
Above FL250 speed is usually referenced to Mach Number and 0.01 would be the smallest increment that a pilot would be expected to fly to.
Below FL250 speed is referenced to Indicated Airspeed and 10kts would be the smallest increment that a pilot would be expected to fly to.
The minimum speed of 250kts for the initial descent (at altitude) would be not far off the mimimum clean speed for a jet aircraft - dependent on weight etc.
Any slower and the aircraft would be approaching the stall speed or would require the use of flap.