1) user object: the date that the next user invoice is due to be issued. prior to this date, other scheduled tasks can add charges to a 'pending' invoice, then on the next invoice date, those charges are totalled up and the invoice becomes 'issued' (i.e. due for payment)
2) radius login object: 'next billing cycle' date is when the access plan accounting period ends - on that date, the plan quota is replenished and the relevant charge/s are added to a pending invoice ready to be issued per (1). There are potentially up to two charges applied at that time: a) monthly fee, b) excess data fee (if relevant)
I hope it offers sufficient guidance - further questions are welcome! Contact us