It has generally good overlap, but both support features the other lacks.
I assume the features of Silver over Apple Swift are well documented in the language extensions section of the docs.
However, while there is a differences and limitations section it inaccurately implies that Apple Swift supports Type Extensions “(a) adding new fields or stored properties” and is incomplete:
I have been submitting bugs for the deficiencies of Silver with respect to Apple Swift, and I list the currently outstanding ones below (to my knowledge none of these are mentioned in the docs currently):