It means the competition organizer has not registered the show with the national organization. This means they have not paid the associated fees and if they are using the national tests, it makes the show illegal.
However, if the non sanctioned show is using tests other than the national tests it is legal.
So in Canada, anyone using the EC tests listed on Dressage Canada's website must have paid the associated fees and licensing. This means the show will be listed as either "Bronze" "Silver" "Gold" or "Platinum". These show organizers will have been provided with e username and password giving them access to the tests without the watermark.
If you ever see a show which is not listed as either "Bronze" "Silver" "Gold" or "Platinum" and that is using the EC tests, OR any show which is using the watermarked tests, the show is most likely non-sanctioned and they should be reported to the EC.
They say money doesn't buy happiness -- well happiness doesn't buy horses!