QuickTime provides a set of APIs named "Windows API", as described here.
Its description tells the APIs are used for Windows, but the references also says the APIs are available on OS X 10.3 and later. The function of those APIs are mainly thread synchronization or thread scheduling.
Why those APIs marked "for Windows" are available on OS X? Are they intended for writing cross-platform code?
These functions are necessary on preemptive multitasking environments. When QuickTime was introduced, Apple didn't have a preemptive OS (besides A/UX and A/ROSE). When ported to Windows NT, they added these functions, so were called 'Windows API'. Later, with Rhapsody and MacOS X, Apple maintained the same functions (but i think there are some alternatives now). Yes, the whole QuickTime API is cross-platform.