Square Cut O-Rings are exactly like normal O-Rings, except that their cross-sections are square instead of circular. This design increases their working pressure, and form fit more snugly in some spaces.
Square Cut O-Rings have been largely phased out by elastomeric sealing manufacturers since the invention of the Quad Ring, or commonly also called a Q Ring or X-Ring. Square O-Rings are still available from some manufcturers, but they typically require tooling charges and/or large quantity orders to produce them.
The Quad Ring has replaced the Square Cut in most cases. The four-lobed design not only provides lower friction than the Square Cut O-Ring, but also, due to its square cross section, it resists spiral twist. When the Quad Ring is squeezed upon installation, they seal with the 4 small contact surfaces on the top and bottom. This also creates a lubricant reservoir formed between the sealing lips that improves performance when startup pressures are exerted.
If your application uses a Square Cut O-Ring, consider switching to the Quad Ring. The Quad Ring offers many advantages over the Square Cut. The AS568A sizing is completely interchangeable with O-Rings, Quad Rings and Square Cut O-Rings.