If you have a configured option named "Bandwidth", then this will override the limit set on the product's "Other" tab. This custom limit will be used in the overages calculations.
For example: If the soft bandwidth limit is set to 1 TB and the configurable option set to 5 TB, the client will only be charged for overage once their usage passes above 5 TB.

Valid bandwidth units are "MB", "GB", "TB". A valid configurable option value would be "100000 MB", "2 TB" or "4000 GB".

In the following example, the user can select during ordering the bandwidth package:

The new bandwidth limit is stored in the client product when the daily WHMCS cronjob is executed. To check the correctness of the configuration in advance, please read this article.