-= (Subtract EQUALS) (Transact-SQL)
Subtracts two numbers and sets a value to the result of the operation. For example, if a variable @x equals 35, then @x -= 2 takes the original value of @x, subtracts 2 and sets @x to that new value (33).
Returns the data type of the argument with the higher precedence. For more information, see Data Type Precedence (Transact-SQL).