A plan is really what defines how things are billed – what distinct resources the user has signed up for, what actual amounts are going to be billed (via the fees, bands, and peaks attached to the plan), when they going to be billed (via fees), and so on.
So a ‘plan’ is the concrete thing to which a user can subscribe or ‘sign-up’. A plan can be understood as a combination of periodic fees, one-off fees and usage based charging schemes.