There was a technical definition dating back to 1977, for a Coupe it was defining leg- and headroom in the rear, even the number of doors and windows and more ISO 3833-1977

I once had the link to a free download, can't find it anymore. Volvo had two- and four door sedans, while the 262C was a coupe. Then there were e.g. two-door BMW sedans which were sold as coupes. Also, four-door coupes have been introduced, so it is quite obsolete anyway