What is the difference between BFRC and TRR rating?

They’re two competing certification schemes that do the same job. BFRC provides third-party certification of a window’s total thermal performance, and Certass does the same through its Thermal Rating Register (TRR). Both rate whole windows on the familiar A-to-G / A++ scale, both are recognised for demonstrating Part L compliance, and both assess the same fundamentals — U-value, solar gain and air leakage. The practical difference is mostly administrative: BFRC is the longer-established and more widely recognised “rainbow label” scheme, run by the British Fenestration Rating Council; TRR is run by Certass and is often the natural choice for installers already registered with Certass for their competent-person certification. For compliance purposes a rating from either is equally valid — the choice usually follows whichever body the installer or fabricator is already signed up with.