A spool, aka cocktail pool, is just a small pool, usually with a spa or a hot tub attached to it. The spool is used primarily for recreation, relaxation and socializing. They are  typically just 3’-4’ deep and with seating for people to sit, talk and enjoy their drinks. A plunge pool is designed to be more active. They are 4’-6’ deep so that you can perform aqua aerobics, water exercises, hydrotherapy, etc.