An asymmetric characterization of the congruence frame

The functor from regular biframes to frames, taking first parts, is shown to be faithful. This result is used to provide many examples of identical embeddings which are epimorphisms in the category of frames. Then the congruence frame, regarded as a biframe, is characterized as being the unique regular biframe extension. This provides a pointfree analogue to a result of Salbany (1970, 1974 [16]) that the forgetful functor from completely regular bitopological spaces to all topological spaces, taking first parts, has a unique section.