If all that determines whether it is a scabbard or sheath is whether it goes to a knife or a sword then the Question "When does a knife become a sword?" becomes all important and not a question to be ignored. It adds so much complexity to the matter that it then becomes nearly impossible to figure out to any degree of real certainty. But if a sheath is a sheath because of it's features. And a scabbard is a scabbard because of it's features. and while these may make them better for a knife or sword, it doesn't make them exclusive to those. Then it becomes a lot easier.