There are 22 letters in the Hebrew Alphabet.
Tav is the 22nd and last letter.
When Ezekiel has his 6 visions, it was over a 22 year period of time, ending when Ezekiel was 52
One of the visions was the destruction of the 1st temple, and the angels went around and wrote Tav on all those to be saved, and the rest were chaff to be burned in the eternal fires.