What is meant by "up to XX flexjets"

There are jet ports in every Beachcomber hot tub. The jets in your tub are customizable-by that we mean, after you receive your tub should you decide that a certain jet is preferred over another, you can customize your tub by purchasing your favourite jet(s). The massage effect with each jet is different. Some jets provide a pinpoint or targeted deep tissue massage using a single stream of water and air mixture. Others offer a spinning massage effect while others offer a more full body vigorous massage effect.