Document tier level statuses

[jeffblake]

request - item is sold out, but the 'request' mode is enabled. Request mode is like joining a wait list. Tickets can be checked out only through to the 'request' checkout step. At that point, an email confirmation is sent to the customer, and IF any more tickets become available, a link will be emailed to them to complete the checkout process and pay (if necessary).
selling - there are available tickets, so the order is eligible to go through to the complete step.
sold_out - no tickets available, request mode not enabled
not_on_sale - each ticket has an on_sale_at and end_sale_at property. This means the current time falls outside of those bounds, so ticket is not eligible to be checked out. I believe miele automatically cuts bookings 24 hours before the scheduled class.
reserve - similar to request, when the item is in stock, but not currently on sale, and 'reserve' mode is enabled. Miele does not use this

I should note that if an order contains any items that are 'request' or 'reserve', then the order cannot be fully completed, i.e. you will not see a 'complete' string included in the order.checkout_steps array.