A three-gun battery firing two of its guns with five-second intervals between rounds and one gun remaining at the ready in case of a misfire; such a battery would be used at an Armed Forces Full Honors Funeral, or for State Arrival Ceremony of a foreign dignitary at the Tomb of the Unknowns in Arlington National Cemetery.
Saw an anon explain that all that makes it a presidential salute versus non is number of shots. 21 shots= President. Biden got 21.
They are wrong.
21 gun salutes are for The President of the United States, former Presidents and Presidents-elect.
AND
Chiefs of state, heads of government and reigning monarchs.
Also, the number of guns and the interval DO matter:
https://en.wikipedia.org/wiki/21-gun_salute#United_States