This sets the outer S-VLAN (service VLAN) ID used to wrap subscriber traffic when eponSrvProfilePortVlanMode is set to qinq(2) for this service profile, per the object's name and EponVlanId syntax (the description is only the placeholder 'Description.'). This is the specific VLAN ID the operator's aggregation network actually switches or routes on for that subscriber's traffic once it's double-tagged, so it needs to be unique per service group at the aggregation point even though the underlying customer C-VLAN it wraps might duplicate other subscribers' VLAN IDs. Setting this to S-VLAN 2001 for a given service profile means every subscriber's traffic using this profile gets wrapped in outer tag 2001 as it crosses into the aggregation network, regardless of what inner C-VLAN each individual subscriber uses.