%= (Modulus Assignment) (Transact-SQL)
Divides one number by another and sets a value to the result of the operation. For example, if a variable @x equals 38, then @x %= 5 takes the original value of @x, divides by 5 and sets @x to the remainder of that division (3).
expression %= expression
Is any valid expression of any one of the data types in the numeric category except the bit data type.
Returns the data type of the argument with the higher precedence. For more information, see Data Type Precedence (Transact-SQL).
For more information, see % (Modulus) (Transact-SQL).