The Frigidaire O-ring 218720100 is used on the filter cup on all refrigerators 26 cubic square feet or less that use filter models WFCB and WF1CB, Puresource and Puresource Plus water filter. Your O-ring should be replaced if you notice any damage to the existing O-ring to ensure a water tight seal.
If your refrigerator is over 26 cubic square ft you should use o-ring part number 218904301.