SIX is an alternative extension of the SIXEL bitmap graphics format. It was developed for terminals and printers by Digital Equipment Corporation (DEC). SIX actually means six pixels of which an image stored in the format consists. If you have a SIX file you want to view but you do not have an access to a DEC terminal, you need to convert it to a more popular format.

VIPS is a format used by libvips internally for calculation but it can also be used for storing images. It is simple, fast and has no size limit. However, only a few other programs are able to open such files which is why it is better to convert them to a more popular format.