Ops Manager Backups - HEAD DATABASE

I am setting up database backups via Ops Manager. I have a 3 node cluster : Replica Set for the Application and Backup databases. In addition 2 Ops Manager Application servers.

I am confused on which host should be defined for the HEAD databases and where the dedicated disk partition should be created.

Is it to be located on the Ops Manager Application host OR the Ops Manager Application database(replica set) host ?

Hopefully this documentation would provide some guidance: