SCTT
Two spring-loaded lips facing the same direction in a single case. Used in a single purpose, severe duty environment as a excluder or for fluid/ lubricant retention.
Opposed
SCTF
Two spring-loaded lips in a single case with faces exposed on each side of the case. Used when positive sealing is required from two directions or when necessary to seal/separate fluids
SCTB
Two spring-loaded lips in a single case with seal backs exposed on each side of the case. Used when a large amount of lubricant is required due to inability to re-lubricate.