ATC can also issue speed restrictions that tell you what not to exceed:

ATC: [aircraft identification] DO NOT EXCEED [speed] KNOTS/MACH [Mach number].

This is an absolute ceiling on your speed. For example, "DO NOT EXCEED MACH POINT EIGHT-FIVE" means your Mach number must stay at or below 0.85.

These restrictions are typically issued to prevent shock stall or overspeed conditions at high altitude, or to maintain separation spacing during the descent.