I am going to training this week, so I thought I would look into this question. Below is from the US Aeronautical Information Manual (AIM) Chapter 4.
4-4-11. SPEED ADJUSTMENTS
a. ATC will issue speed adjustments to pilots of radar-controlled aircraft to achieve or maintain required or desire spacing.
b. ATC will express all speed adjustments in terms of knots based on indicated airspeed (IAS) in 10 knot increments except that at or above FL 240 speeds may be expressed in terms of Mach numbers in 0.01 increments. The use of Mach numbers is restricted to turbojet aircraft with Mach meters.
c. Pilots complying with speed adjustments are expected to maintain a speed within plus or minus 10 knots or 0.02 Mach number of the specified speed.
I'm uncertain of its applicability outside of the USA.