A TIFF can contain several pages. You can load image or parameters from a page using TIFF_ImageIndex (or just ImageIndex). Each page can also contain sub-images, so you can load a specific sub-page using TIFF_SubIndex.
This is useful, for example, to load an embedded JPEG from a DNG (camera raw format), which is located in the first page and the second sub-index.
// Load the embedded Jpeg from a DNG ImageEnView1.IO.Params.TIFF_ImageIndex := 0; ImageEnView1.IO.Params.TIFF_SubIndex := 1; ImageEnView1.IO.LoadFromFileTIFF('DCS001.DNG');