The InstrumentLeg component block, like the <Instrument> component block, contains all the fields commonly used to describe a security or instrument. In the case of the InstrumentLeg component block it describes a security used in multileg-oriented messages.
Several multileg-oriented messages specify an <Instrument> Leg component block. An instrument can have zero or more instrument legs. The fundamental business rule that applies to the multileg instrument is that the multileg instrument is defined as the combination of instrument legs. The multileg instrument must be able to be traded atomically – that all instrument legs are traded or none are traded.
The LegRatioQty <319> is used to define the quantity of the leg that makes up a single unit of the multleg instrument. An option butterfly strategy is made up of three option legs.