Environmental Protection Agency (EPA)
The federal agency setting vehicle and engine emissions standards. Its rules bind manufacturers, so they reach every fleet through the vehicles available to buy.
EPA emissions standards apply to the manufacturer rather than to the fleet operator. A public agency does not comply with them directly; it experiences them through what is on the market, what the aftertreatment systems require, and what the engines cost.
That indirect route is why emissions rules show up in a fleet as maintenance and specification issues rather than as compliance paperwork: aftertreatment systems to service, diesel exhaust fluid to stock, and vehicle categories defined by regulation appearing in bid documents.