BITMAPINFOHEADER.biClrUsed Field

If iClrUsed 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, the biClrUsed member specifies the size of the color table used to optimize performance of the system color palettes. If biBitCount equals 16 or 32, the optimal color palette starts immediately following the three DWORD masks.

When the bitmap array immediately follows the BITMAPINFO structure, it is a packed bitmap. Packed bitmaps are referenced by a single pointer. Packed bitmaps require that the biClrUsed member must be either zero or the actual size of the color table.