The B major seventh flat sixth chord is a 4-note chord consisting of the notes B, D#, G and A#.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as BM7b6.
Interactive piano diagram for the B major seventh flat sixth chord
Chords that are a subset of BM7b6. These contain less notes but all of them are included in BM7b6.