A men's swim boxer fits like a brief but covers the top of the thigh like a short. It's the compromise for those who find the brief too revealing and the short too bulky for swimming, and it's allowed in pools that ban shorts.
Most public pools ban swim shorts for hygiene reasons and accept the men's swim boxer like the brief. The difference between the two is coverage and drag: the brief offers maximum freedom of movement and remains the choice for training, while the boxer provides more coverage while still being fitted, which is suitable for regular leisure use.
Three points. The anti-chlorine treatment, mentioned on the label: this is what separates a men's pool swim boxer from a beach model. The long men's swim boxer, which goes lower on the thigh, also exists, as do luxury versions of the men's swim boxer with more refined finishes. The integrated lining, which replaces underwear and avoids the need to wear one under the swimsuit. And the waistband, which should be a wide flat drawstring rather than a round lace, as a round lace can slip once wet. The size is measured around the hips and should fit snugly when dry, as the material stretches in water.