Mediator is a "smart" relay, in that it knows the adressee and can relay with that in mind.

Relay, on the other hand, does not know the destination, it works deterministicly. Iff that relay would always route messages to Bob, then Alice doesn't need to tell anything specific to the relay.

Horn, S. R., Weston, S. J., & Fisher, P. (2020). Identifying causal role of COVID-19 in immunopsychiatry models [Preprint]. PsyArXiv. https://doi.org/10.31234/osf.io/w4d5u