Sub-orbital is just a word. Like most, if not all, words, it doesn't really mean anything. It's just a string of vocal sounds or characters, based on the Germanic language of English, which evolved from Latin, invented by a tribe called the Latins. The Romans agreed to adopt the language and have a royal marriage to avoid conflict, then the Romans conquered much of the world, and Latin's influence has not been forgotten. All the Romance and Germanic languages have been based off of them, including English, Spanish, Portuguese, French, German, etc.
"Sub" is a prefix that used to be a preposition in Latin meaning "under." It would have been used like "under the bridge" or something like that, and it became a prefix later. "Orbit" comes from Middle French "Orbite," with the same meaning, which came from the Latin "orbita," which originally referred to the track of the wheel. Then it was used to mean celestial orbit, when this started to become important in religion, as the mediaeval people believed that everything was in orbit of the Earth.
The point is, these are all just words. They don't mean anything. What matters is the nature of reality. Words are a human convention.
Remember, my friends: Triviality in all things.