With Red Hat Satellite 6.10 the subnet and generic host images have been removed:

https://docs.redhat.com/en/documentation/red_hat_satellite/6.10/html/release_notes/release_information#deprecated_functionality 

Red Hat Satellite 6.11 documentation re-introduced the subnet image.

https://docs.redhat.com/en/documentation/red_hat_satellite/6.11/html-single/provisioning_hosts/index#Creating_Hosts_with_PXE_less_Provisioning_provisioning

Subnet image can be found up to Satellite 6.17 documentation.