The flat face of the fitting connects to a tube adapter so you can slide the tubing and fitting sideways to disconnect in cramped spaces. They are also known zero-clearance fittings.

A tube adapter sits flush against the flat face of the fitting so you can slide the steel tubing and fitting sideways to disconnect in cramped spaces. Also known as zero-clearance fittings.

The sleeve in these fittings is rubber instead of metal, so it cushions the tubing and absorbs vibration while creating a tight seal.