g77 supports certain constructs that have different meanings in VXT Fortran than they do in the GNU Fortran language.
Generally, this manual uses the invented term VXT Fortran to refer VAX FORTRAN (circa v4). That compiler offered many popular features, though not necessarily those that are specific to the VAX processor architecture, the VMS operating system, or Digital Equipment Corporation's Fortran product line. (VAX and VMS probably are trademarks of Digital Equipment Corporation.)
An extension offered by a Digital Fortran product that also is offered by several other Fortran products for different kinds of systems is probably going to be considered for inclusion in g77 someday, and is considered a VXT Fortran feature.
The -fvxt option generally specifies that, where the meaning of a construct is ambiguous (means one thing in GNU Fortran and another in VXT Fortran), the VXT Fortran meaning is to be assumed.