Parc ferme is an enclosed and secure area in the paddock where the cars are weighed and any other checks deemed necessary by race officials are made. Teams must leave their cars here from 1830 on Saturday until 0830 on Sunday.
However, the cars are deemed to be under parc ferme conditions for a much longer period - from the time they first exit the pits during qualifying until the start of the formation lap immediately prior to the race.
Under these conditions, the work teams may carry out on their cars is limited to strictly-specified routine procedures, which can only be performed under the watchful eye of the FIA Technical Delegate and race scrutineers. Fuel may be added to the cars (those eligible for the final period of qualifying may only replace what they used in that period), tyres changed and brakes bled. Minor front wing adjustments are also allowed, but little else. These controls mean that teams cannot make significant alterations to the set-up of a car between qualifying and the race.
The only exception to this is when there is a "change in climatic conditions", for example a dry qualifying session followed by a wet race, or vice versa. In this case the FIA will give the teams permission to make further appropriate changes to their cars.
Should a car require an engine change between qualifying and the race, then the driver concerned will be required to start from the back of the grid. Modifications to other parts or suspension set-up will require the driver to start from the pit lane.