NIST REFPROP 9 is a reference fluid thermodynamic and transport properties database developed by the National Institute of Standards and Technology (NIST). Here's a piece of information about it:

REFPROP is a comprehensive database of thermophysical properties of fluids, including refrigerants, hydrocarbons, and other industrially important fluids. It provides accurate and reliable property data for use in simulations, design, and optimization of various engineering systems, such as refrigeration and air-conditioning systems, power generation and conversion systems, and chemical processes.

You're referring to NIST REFPROP 9!