The Less Than or Equal To function block evaluates input A to see if its current value is less than or equal to input B.
If input A is less than or equal to input B: power will flow through the function block.
If input A is not less than or equal to input B: power will not flow through the function block.
According to the above example:
If MI 1 value is greater than the MI 3 value; then MB 51 will go to logic "1" (ON).
If MI 1 not greater than the MI 3 value; MB 51 will go to logic "0".