Output limiter (出力リミッター Shutsuryoku rimittā), also known as Ability restriction (能力限定 Nōryoku gentei), is a magical method used by the Administrative Bureau to ensure that a mage will not channel more magical energy than permitted, effectively reducing their mage rank by several levels. Lifting a power limiter requires authorization by a high-ranked TSAB official, and the release can be additionally limited to certain time.
The Administrative Bureau has the legal authority to impose power limiters upon mages as means of protection, to prevent them from overstraining and damaging their bodies with magic. For instance, Lutecia Alpine has a powerful limiter placed upon her after the JS Incident, presumably to prevent her from inadvertently using her extremely potent summoning magic. It is temporarily lifted from her in ViVid during the training mock battle.
Output limiters are also used by the Bureau personnel themselves to circumvent aggregate mage rank restrictions that ensure that no single unit can "amass" too many powerful mages in its ranks (similar how each unit has a limited materiel allowance). Limiters provide a convenient loophole around these regulations, and the Bureau generally turns a blind eye on it. For example, Riot Force 6 managed to go way over the mage rank quota by having limiters placed on all three of its over-S mages, temporarily reducing them to A (Hayate Yagami) and AA (Nanoha Takamachi and Fate T. Harlaown) ranks. Hayate's limiter in particular required simultaneous authorization by Carim Gracia of the Saint Church and Admiral Chrono Harlaown of the Dimensional Navy to lift.
Devices have a functionality similar to power limiters, as their functions can be permanently locked and unlocked with sufficient authorization. Cross Mirage, for instance, had its Dagger Mode (and, presumably, the Blazer Mode) locked away by Nanoha until Teana Lanster has mastered its Guns Mode.