In short, you can use just about any kind of formula that evaluates to
either TRUE or FALSE, or, returns a numeric value. A formula that returns a
numeric value of 0 evaluates as FALSE, *any* other numeric value evaluates
as TRUE.
When the formula evaluates to FALSE, 0 or an error (like #N/A) the format is
not applied.
When the formula evaluates to TRUE or *any* number other than 0 the format
is applied.
The only limitation that I can think of is you can't use formulas that
contain array constants and the total length of the formula must not exceed
255 characters. However, you can get around those limitations by using
defined names.
Another interesting thing is that formulas used to apply conditional
formatting are automatically processed as array formulas.