These include (p,V, T) measurements, saturated density measurements, and any other measurements which give information on the (p,V, T) surface of a fluid.
Source:
PAC, 1981, 53, 1805. 'Assignment and Presentation of Uncertainties of the Numerical Results of Thermodynamic Measurements' on page 1818 (https://doi.org/10.1351/pac198153091805)