Generally, the start file for WorldView-3 is the .imd file, as stated here: https://www.pcigeomatics.com/geomatica-help/references/gdb_r/WorldView-3.html. However, the Spectral Index documentation at https://www.l3harrisgeospatial.com/docs/SpectralIndices.html#Run states that the image used for spectral indices must contain wavelength metadata. Image data in general is contained within the WorldView-3 .tif files.