With flutes having a C foot joint, the lowest possible note is a C1.
On flutes with an B foot joint, you can play a half-step lower, down to a B. The B foot joint is heavier and longer than the C.
An additional key on the B foot joint, called the Gizmo, prevents the highest C (C4) from sounding too dull.
The advantage of an B foot joint is that especially the high notes in the third octave sound less bright and piercing.
Most flutists prefer a C foot joint because they feel that the low notes with an B foot joint sound too low and it takes longer for the flute to warm up.