LUFS is absolute loudness referenced to full scale; LU is the relative loudness difference

EBU R 128 uses two related units introduced by ITU-R BS.1770. LUFS (Loudness Units referenced to Full Scale) is an absolute measure: an integrated LUFS value states how loud a programme is relative to digital full scale (so the -23 LUFS target is an absolute level). LU (Loudness Unit) is the relative complement, used to express differences and tolerances — a programme at -25 LUFS sits 2 LU below one at -23 LUFS, and every R 128 tolerance is stated in LU. LUFS is equivalent to LKFS (the name used in ITU-R BS.1770); the EBU adopted LUFS to match international naming conventions.

Examples

Programme A: -23.0 LUFS (on target). Programme B: -25.0 LUFS (2 LU below A). A tolerance quoted as +/-1.0 LU is a relative window around the -23.0 LUFS absolute target. LKFS and LUFS name the same quantity.

Assessment

A mix is at -20 LUFS. How many LU is it above the -23 LUFS target? Which of LUFS/LU is absolute and which is relative? Is LKFS a different unit from LUFS?