333068bb_Structure_BITMAPINFOHEADER__2
[0] biClrUsed [1] DWORD [2] 
Specifies the actual number of color indices in the color table used by the bitmap. If this value is zero, the bitmap uses the maximum number of colors corresponding to the value of the biBitCount member for the compression mode specified by biCompression. If biClrUsed is nonzero and the biBitCount member is less than 16, the biClrUsed member specifies the actual number of colors the graphics engine or device driver accesses. If biBitCount is 16 or greater, then biClrUsed member specifies the size of the color table. This can be used to optimize performance of Windows color palettes. If biBitCount equals 16 or 32, the optimal color palette starts immediately following the three doubleword masks. If the bitmap is a packed bitmap (a bitmap in which the bitmap's pixel data array immediately follows the BITMAPINFO header and which is referenced by a single pointer), the biClrUsed member must be either 0 or the actual size of the color table.
Structure BITMAPINFOHEADER
2