This is the Relative Name (Rn) of a VSAN object, representing a Fibre Channel virtual SAN configured on the UCS fabric interconnect, the naming component that distinguishes this instance from its sibling Vsan objects under the same parent in the UCS tree. Admins rarely read the Rn directly, but UCSM uses it to build the full Dn path, so it matters when correlating which parent object a given Vsan instance belongs to. For instance, if two Vsan objects sit under the same parent container, their Rn values are what tell UCSM (and any script parsing the Dn) apart which one is which.