In a deadweight tester, the force is supplied by a mass in a gravitational field on top of a piston with a specific cross-sectional area that rotates within a cylinder. This piston cylinder system is manufactured to very tight tolerances for smooth, precision operation. The fluid (pneumatic or hydraulic) under the piston is pressurized so the piston and mass “float” in equilibrium. The pressure in the fluid is equal to the force exerted by the mass over the cross sectional area of the piston.
It's the primary instrument you are using as a reference to measure pressure in a standard way. They usually refer to instruments such as deadweight testers, pressure balances and piston gauges, which are traceable to SI units. Using any of these devices is essential to a fully equipped pressure calibration lab.