Hello guys, in the below link the statement:

If you have deployed a Red Hat Ceph Storage system separately and then used director to deploy and configure OpenStack, before you can upgrade your Red Hat OpenStack Platform (RHOSP) deployment from version 16.2 to version 17.1, you must upgrade your Red Hat Ceph Storage cluster from version 4 to version 5, or from version 5 to version 6. After you upgrade your Red Hat Ceph Storage cluster, Red Hat OpenStack Platform 16 continues to operate and be compatible with the storage cluster. 

it's contradictory.

Ceph cluster version 6 it's not compatible with RHOSP 16.2, so it should be clear from the docs WHAT VERSION of RHCS is compatible with RHOSP 16.2 and 17.1. 

And what would be the high level path to do a ffu with an external ceph cluster 4,5,6 .

FFU DOCS