For: Operators using Hub
In Hub, go to EMP / Fleet → Spending limits (/spending-limits).
New rules are drafts. To turn a rule on you use Activate and the form toggle Enforce spending limits.
Rollover is labelled Roll unspent budget over (for N months, up to a € cap). The list can show “(+1 month carry)”.
This article describes the page chrome. It does not say a rule will refuse a charge — that is not confirmed as live.