Top up is a feature included in Works mobile subscriptions.
The service allows the user to renew his data up to a total of four times if the user has used all the included data for a given billing period. You get that at a reduced price.
The alternative is a so-called speed drop, where we turn down the data transfer speed (the user experiences that mobile data does not work). You will then have to adjust the user's data pack yourself.
A user has Works 3 GB. The user spends all his data on streaming without thinking about it at the beginning of the month. This means that the user gets a top up of his data pack of another 3 GB. The top up is posted on the next bill with date/time as an extra service. The user therefore has 6 GB data to spend in that month. In fact, the user can have 12 GB in addition to the original package if he is topped up four times - i.e. a maximum of 15 GB in total.
If so much data is used, you might want to consider a larger basic package for the user. But you cannot, for example, choose a basic package with 50 GB data. However, usage of 100 GB in one month is possible if you have a basic package of 20 GB and top up four times.
You may unsubscribe users' top up, both on existing numbers and future creations, if you want.
Re-registering a user to the top up service is of course also possible. However, this will only work from the next billing period. In other words, it can be bad for unsubscribe people with 20 GB, as they really cannot get more data in a given period if they are unsubscribed and the first 20 GB are spent. If this is the case you will need to make contractual changes with your account manager