Your Primary subscription is the subscription you need to maintain to retain "active" student status with MSIA, which allows you to access events not open to the public and to maintain active Initiate and Minister status.
By default, your Discourse subscription is your Primary subscription, unless you've finished all 12 years, in which case your SAT subscription becomes your Primary subscription.
That Which Is is an optional, supplemental subscription that has no impact on your status as a student, so the Primary designation doesn't apply to it.