Stretch is a generic term. If someone were to just say "I stretched my bike 4 inches." They would almost certainly mean that they lengthened the swing-arm by 4 inches. If someone had more extensive modifications done, like a hard tail, they would probably clarify further. Whether or not someone lengthens the top or bottom tube on a hard tail is going to vary by application; the person would probably make that clear or the photos would for them. In other words, there is no way to know unless you are told or shown.