Callback function question

All,
I have this API function which requires a callback function:

Function midiInOpen(lphMidiIn:Int Var, uDeviceID:Int, dwCallback:Int ,dwInstance:Int, dwFlags:Int)

The callback function:

Function MidiCallBackBM:Int(hmIN:Int, wMsg:Int, dwInstance:Int, dwParam1:Int, dwParam2:Int)"Win32"

The above callback function works.

But, if I remove the "Win32" bit at the end of the function declaration, I get:


The questions:
- what is that "Win32" for ?
- When and where to use it ?
- Is there any documentation to read about ?

Thanks for your attention,
Sergio.