The HANDLE type is a 32-bit or 64-bit integer that is used to represent a block of memory, or a generic "pointer" to an entity that DTWAIN internally will keep track of.  


Usually in DTWAIN, this block of memory describes a Device Independent Bitmap, or image data.  In other contexts, a HANDLE is also returned by functions such as DTWAIN_SysInitialize.


If the application is using dtwain32.dll or dtwain32u.dll, the HANDLE type will be a 32-bit integer.  For dtwain64.dll and dtwain64u.dll, the HANDLE type will be a 64-bit integer.