This is what I found
The SFI Foundation is a non-profit organization established to issue standards for the performance and racing industry. SFI tests a long list of products including gloves, neck braces, harnesses, shoes and uniforms.
The SFI specification for uniforms, spec 3. 2A, rates how long a driving suit will offer protection from a second degree burn in an approximation of a raging gasoline fire between 1800 and 2100 degrees. SFI ratings are calculated in calories per unit area per time of exposure. To figure a uniform's approximate protection time until second degree burns, divide the Thermal Protection Performance (TPP) number by 2.