Subscriptions
Subscriptions is who pays you, and which plan each of them is on. Stripe takes the money; here you read every subscription, correct what a plan opens for someone, and put a customer on a plan without paying.
The walk
The room
Open Subscriptions to see everyone on a plan: who they are, which plan, and since when.

Choose Active or Cancelled to see only the subscriptions in that state; a cancelled one keeps its row here, marked cancelled.

One subscription
Open a subscription to see its plan, its status, and how they pay: billed by Stripe, comped by you, or granted by a relay.

Choose Correct the record… to change their plan or status: it changes what they can reach, never what they're charged.

To stop someone's billing, cancel the subscription in Stripe; nothing in this room moves a payment.
Comp a plan
Choose + Comp a plan to put someone on a plan without paying: the email they sign in with, and the plan.

Choose Grant the plan: it becomes a real subscription, tagged comp in the list, and it ends the way any other one does.
What it refuses and why
These are the console's own words when a change can't be saved or a request can't go through.
a valid email is required
A plan is held by the email a customer signs in with, so it has to be an address they can receive mail at.
How plans are sold and what they open: Payments.