I thought now I can manage all my postpaid lines. Why are there Postpaid - Supplementary Line under the line of business?
You are correct that do not need to sign up under this line of business because signing up as Principal Line would allow you to manage all your postpaid lines including the supplementary lines. This sign up is to allow the supplementary line holder to have an overview and to manage his/her own line. He/she would not be able to view the Principal or any other Supplementary accounts.