[Exchange] Need help deciphering terminology

[RabbieBurns]

Im trying to set something up and its not working, and the advice Ive been given from Microsoft begins:

 

“The on-prem MBX needs to have a proxy that matches the cloud authoritative accepted domain ..."

 

Now I'm not entirely sure what that means.. Does it mean that in our on premise Exchange, I need to add in the accepted domains section, the external cloud domain ?