This read-write IP address identifies the MLPPP discriminator, a value used to identify which physical links belong to the same multilink bundle, and can only be set when the bundle is configured as multilink. Admins configure it during multilink bundle setup so that link-negotiation on each member link resolves to the correct bundle rather than forming a separate one. For example, an admin might assign the router's own management IP address as the discriminator shared across all links intended to join one multilink bundle.