What is Meant by a Cover (Retainer) Plate and Why is it Required with an All-Rubber Split Seal?

A cover plate is a piece of metal (generally) that is installed after the oil seal has been placed into the housing bore. The cover plate pushes on the back side of the oil seal and compresses it into the housing. A cover plate is required with an all-rubber split seal (such as the Garlock Split Model 23) because this style does not have an outer reinforcement and must be compressed inside the housing to ensure it stays in place.