Section '3.2. Preparing a Satellite-registered system for the upgrade' of the docs, Procedure step 3:
Enable and synchronize all required RHEL 8 and RHEL 9 repositories on the Satellite Server with the latest updates for the source and target OS versions.

`x86_64 8 or `

Instead of:

`x86_64 `

As an example, in case the Satellite registered host is 8.6, if the generic `8` version of the repository is selected, when the host performs the release set (section 3.1 step 5) it will fail with an error:

1. subscription-manager release --set 8.6
   No releases match '8.6'. Consult 'release --list' for a full listing.

This might work if the host is registered to the CDN but it won't with Satellite.

Reported by: rhn-support-rfurlan

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/9/html/upgrading_from_rhel_8_to_rhel_9/assembly_preparing-for-the-upgrade_upgrading-from-rhel-8-to-rhel-9#annotations:a4333988-0241-4c7a-a70f-77f57a0daf74