You can set a data usage limit in the Threshold field. Each group can have their own threshold, specified as a MB value. If you select Cap at threshold, all data is capped once the threshold is reached, with the exception of sites and apps explicitly allowed in your allow list.
The cap Reset interval determines when the data count resets to zero. It can be set to daily or monthly, and for monthly plans, a specific Reset day can be chosen.
Excluding the Global (or "default") group, the cap can also be disabled. When this is done, notifications are not sent to admins or users, and the Jamf Trust app data usage meter will indicate that no cap applies (effectively the device will operate with an "unlimited" allowance).