Flaps up maneuver speed in the 737 is Vref40+70. Above FL250 Boeing recommends using Vref40+100 as an approximate min/holding speed to provide adequate margin to buffet.
Not sure if this is what you're referring to - I've only ever been asked to fly "minimum clean" at low altitude (in which case we just fly Vref40+70), if we're high they'll normally say "slow to minimum speed", in which case we maintain a minimum 10 knot margin above the amber on the speed tape.
Edit: I see you're specifically asking about the significance of FL250. I'd say this is a level Boeing has chosen above which flying Vref40+70 may not give adequate margin to buffet.