A spa filter replacement number is a number assigned to your filter by the replacement filter manufacturer. Confusingly, it is usually NOT the same as the spa manufacturer's part number and the number is often NOT printed into the filter itself. Yes, this is all as dumb as it sounds and if we were in charge of this stuff, the world would be a better place.
Most hot tub filters are made by one of three companies: Unicel, Pleatco and Filbur (also sold as Clarathon). If you have your spa filter's number from any of these manufacturers, please enter the number (example C-1234) in the box marked 2. If you get no results, try entering the number without the prefix (1234). If you still get no results, no worries. Just use method two, three or four.