The following query returns the result 'TURNERyxX'.
RTRIM removes the individual occurrences of 'x' and 'y' starting from the right side of expr1 argument, and it stops removing characters when it encounters 'X' because 'X' is not a character in the expr2 argument.
SELECT RTRIM('TURNERyxXxy','xy');
The RTRIM function removes individual occurrences of characters in expr2 argument. How these characters are arranged in expr2 argument does not impact the result.
For example, running the following query returns the same result 'TURNERyxX'.
SELECT RTRIM('TURNERyxXxy','yx');