invoicing_threshold
when using the Update Subscription API.
Alerts on subscriptions can be setup to alert when particular thresholds have been reached. See the Alerts API for information on how to setup alerts.
partially_invoiced_amount
field. This field represents the amount that has already been invoiced for that line item as a result of threshold invoicing and so is not included when calculating the amount due on that invoice. See the guide for invoice calculations and the step detailing previously invoiced amounts.