<p>The hierarchical tier that this ACL belongs to.</p>

      <p>
        ACLs can be assigned to numerical tiers. When evaluating ACLs, an
        internal counter is used to determine which tier of ACLs should be
        evaluated. Tier 0 ACLs are evaluated first. If no verdict can be
        determined, then tier 1 ACLs are evaluated next. This continues
        until the maximum tier value is reached. If all tiers of ACLs are
        evaluated and no verdict is reached, then the <ref column="options"
        key="default_acl_drop" table="NB_Global" /> option from table
        <ref table="NB_Global" /> is used to determine how to proceed.
      </p>

      <p>
        In this version of OVN, the maximum tier value for ACLs is 3,
        meaning there are 4 tiers of ACLs allowed (0-3).
      </p>