We just added some info on the Pricing Plan Basics article.

Here's an excerpt:

Recurring vs. Non-recurring

You might be wondering what "None (no recursion)" means. Welp, that's pretty simple, actually. A subscription with a pre-defined recurring frequency will always have a current billing period with a definitive end according to the current invoice billingDatetime. If the subscriber is subscribed to a pricing plan without a defined frequency (the "None" option), then the current period will not have a pre-defined end. In other words, the "current" or "pending" invoice will not have a set billingDatetime. On subscriptions of this type, it is up to you or your system to determine when the current period will end.

You may change the billingDatetime on the current invoice anytime you like, whether the invoice already has a billingDatetime or not. By changing the billingDatetime (or setting it in the case of a non-recurring subscription), you can decide when the current billing period will end and when the customer will be charged for that invoice. You can even set it to now to transact the current invoice immediately.

When the billingDatetime on the invoice passes, a new current/pending invoice is created. Whether or not this new invoice has a billingDatetime, and to what date it is set, depends on the pricing plan frequency. If the frequency is set to Bi-monthly, for example, the billingDatetime for the new current invoice will be set to 2 months after the billingDatetime of the previous recurring invoice. If the frequency is set to "None" (the non-recurring plan type), the billingDatetime is not set. The invoice will sit there and wait until you're ready to execute it.