The best explanatiom has been written by Valerie_cct and here is a copy of her message.
From Valerie_cct elsewhere in this forum in italics:
Solutions used in enemas, among other categorizations, can be classified according to their ability to either draw water out of the body, be absorbed by the body, or be neutral. In that order they are known as hypertonic, hypnotonic, and isotonic.
The small volume disposable squeeze bottle enemas are hypertonic, meaning they draw fluids out of the body into the colon in order to provide the volume of water to promote a bowel movement. These hypertonic solutions can also be very irritating to the rectum and colon, and cause considerable cramping during and for hours after use.
Plain water is hypotonic and is absorbed into the body during enemas and colonics. During a typical enema or colonic session this doesn't present problems to an individual in good health and physical condition, but retaining a water enema, especially for young children and the elderly can cause problems.
A standard saline solution (1 tsp. salt for each pint of water) is isotonic, meaning that it is neither absorbed by the body nor draws fluid out of it. A saline solution most closely matches the electrolyte balance of the fluids in our bodies. Soapsuds enemas also are generally isotonic.
So when retaining an enema, it is usually best to do so when you are using a saline solution. If you retain a plain water enema, it would be a good idea to replenish electrolytes afterwards with something like 100% pure grapefruit juice, Welches 100% pure unsweetened Concord grape juice, or Gatorade or other electrolyte fortified drink.
I hope this helps you understand the difference between solutions.
Happy new Year!