I am not sure what you mean by it just has one argument.
It has 3 conditions
(ISNUMBER(Y!$1:$3)) - only include numeric values in rows 1:3, i.e. not blanks
(Y!$1:$3<=Z!A1) - only include values in rows 1:3 <= A1 on Z
(X!$B$1:$B$3="T") - only include rows where the corresponding value in column B of X is a T
The three conditions are ANDed (*), to give an array of 1s and 0s which are summed.