The variation tier setting configured for the Head Contracts tool only affects the number of tiers used for the overall change management process. An owner can always be marked as a designated 'Reviewer' on a HCV when you want it to be approved by an owner. To learn how to approve a HCV as an owner, see Approve or Reject Head Contract Variations (PCCOs).
If your project is configured for 2-tier or 3-tier variations and your project team wants an owner to review variation information at an earlier point in the overall change management process, you can email a Potential Variation (PV) or Variation Request (VR) to an owner. For instructions see the links below:
When configuring the number of head contract variation (HCV) tiers for the Head Contracts tool, a user with 'Admin' permission on the tool can choose between a 1-tier, 2-tier-, or 3-tier configuration. See What are the different variation tiers?