I can't answer the restrictions/rules part of the query as unfortunately I was unable to utilise my letter. I had to rearrange my next already booked cruise to a date beyond the expiration date on the letter. I did phone customer services and they agreed to extend the date on the letter, and provided an email to support that decision which I printed off and attached to the letter.
Due to medical problems for which I could not secure insurance cover, I had to change the cruise arrangements again, and decided to let the letter lapse.
I don't know the criteria for the issue of the letter, but Mrs J spent every evening in the casino but didn't lose more than £50/£100 a day, and some days ended ahead, the winnings being re-invested the following day!!!