What you might want to do is "test" the M11 code against a one-night stay.

Start on the first (")ind and reserve") page of the Marriott site by trying to reserve a one-night stay. (You'll have to fill in your MR # and "M11" in the appropriate fields).

On the resultant screen either a link to the property will appear (indicating the code is valid) or there will be a disclaimer indicating non-validity.

If it is valid, click on the button indicating you want to see the rates available are for a one-night stay and they (it) will display along with a link to the rate's terms and conditions (which will ispell out any restrictions as well as the cancellation policy).

Don't worry, you're not committing yourself by doing this - you can either back your way out using your browser or click on the "X" in the upper right of the frame.