Exchange resource forest topology concept usually used in enterprise environment. In this topology, we will have account and resource forest. Account forest is where the account created while resource forest is where service provider (Exchange, Lync) that have account disable linked back to account forest.
Usually, this been used while an organization used service provider services while maintaining there own domain on-premise.
For Exchange services, linked mailbox are use in resource forest. Thus, the require for this topology is :
1) DNS – both forest can reachable each other
2) Trust – One-way outgoing trust from Resource to account forest. Can use either forest trust or External trust.
Once these 2 requirement fulfill, we can utilize linked mailbox.