Chroma subsampling notation - rules
Yes, there are.

Let x:y:z is a chroma subsampling. Then


x = 4
So the begin of the notation is 4:.
y|x (y is a divisor of x)
So there are 3 possibilities for the first two numbers: 4:1: or 4:2: or 4:4:.
z = 0 or z = y
So there are 6 possibilities: 4:1:0, 4:1:1, 4:2:0 4:2:2, 4:4:1, or 4:4:4.
y + z determines the resulting color quality (higher is better).
So 4:4:4 is the best (as 4 + 4 = 8) while 4:1:0 is the worst (as 1 + 0 = 1).