offlineUnitRenderAction_Preflight
This is used with offline audio units (of type 'auol'). It is used when an offline unit is being preflighted, which is performed prior to the actual offline rendering actions are performed. It is used for those cases where the offline process needs it (for example, with an offline unit that normalizes an audio file, it needs to see all of the audio data first before it can perform its normalization).
Declaration
static var offlineUnitRenderAction_Preflight: AudioUnitRenderActionFlags { get }