The first BD standard defined 25GB for SL and 2x25GB for dual layer discs.
The newer BDXL standard defines 33GB for SL, 2x33GB for DL and 3x33GB (100GB) for TL discs. The quad layer is defined as 4x32GB.
BDXL of 33GB and 2x33GB is exclusively used by the so called UHD standard for factory pressed discs.
BDXL 3x33GB disc are easily available for consumers. The 4x32GB is used in the XDCAM disc and Sony ODA system.
Later I will post the BD whitepaper where everything is explained in details.