piano chord
Fsus4
F1B♭4C5
The third replaced by the fourth, which leans on the third it is standing in for.
What is true of it
Fsus4 is F, B♭ and C — 1, 4 and 5 counted from F.
These are also the notes of B♭sus2/F and A♯sus2/F. Which name is right depends on what is underneath and what came before — the notes alone do not decide.