The B flat major seventh chord is a 4-note chord consisting of the notes Bb, D, F and A.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as Bbmaj7.
Chords that are a superset of Bbmaj7. The chords include more notes but always Bb, D, F and A.
Chords that are a subset of Bbmaj7. These contain less notes but all of them are included in Bbmaj7.
In case you prefer a non-interactive variant of the chord chart, we've embedded a PNG image below that shows the notes for the the Bbmaj7 chord. Feel free to save or share the image as needed.