The VLAN-ID referring to the private VLAN of the private VLAN domain. A private VLAN is the unique and common VLAN identifier of the whole private VLAN domain. Every port in a private VLAN is a member of the private VLAN. The IP subnet of the private VLAN shall be associated with the private VLAN. Only the private VLAN is visible outside of the domain.
Sources for this description
D-Link official switch MIBsD-Link MIB copies (LibreNMS/Observium Community)