.zip file. Save the file to some kind of portable media, like a flash drive.
certificates.zip file. Unzip the directories until the PEM files for the subscription certificates are available.
import command:
# subscription-manager import --certificate=/tmp/export/entitlement_certificates/596576341785244687.pem --certificate=/tmp/export/entitlement_certificates/3195996649750311162.pem Successfully imported certificate 596576341785244687.pem Successfully imported certificate 3195996649750311162.pem
subscription-manager-gui

.pem file of the product certificate.
