FACTBOX-How airlines play the aviation slot machine

Sept 25 (Reuters) - With the future of Alitalia AZPIa.MI hanging by a thread, rival airlines are eyeing its sought-after take-off and landing rights.

The allocation of slots, the lifeblood of many airlines, has evolved into a multi-million-dollar business with soaring prices. It is a largely secretive system bound by rules for which even airlines are unable to agree a common interpretation.

Here are some details:

-- What is a slot?

The right to land or take off on a specific day and time and to use airport infrastructure needed to operate flights.

-- Which airports operate a slot system?

Airports designated as "coordinated" airports: in practice the most congested. 140 airports worldwide ration flights.

-- Exemptions to the slot system

State flights, emergency landings, humanitarian flights.

-- Who allocates the slots?

Under European Union rules, each country appoints an independent coordinator. It must be impartial, though critics sometimes complain that local airlines get preference.