This is just like Control.Monad.Reader.Class except you can access the context of any Reader in the monad stack instead of just the top one as long as the context types are different. If two or more readers in the stack have the same context type you get the context of the top one.
Documentation
class Monad m => Context x m whereSource
Same as MonadReader
but without functional dependency so the same monad can have multiple contexts with different types