What is Mode 2  (M2) on CD

Mode 2 is a data mode designed to be able to store more data per block.  Instead of only 2048 'user data' bytes these blocks can store 2336 bytes per block. 
As a result these blocks don't have a third layer error correction.

If extracted raw you end up with 2352 bytes per block.  If extracted as 'user data' you end up with 2336 bytes per block.

This mode is actually never used and also know as Mode 2 Useless.  There's no support for it in File Systems either !