The Swiss arms 45 degree mount (also referred to as an offset mount) is a utility block for building the perfect optics/accessory setup for your gun! Suitable for any 20mm RIS/RAS rail, it is best suited for rifles e.g. Assault Rifles, SMG's, PDW's etc.
Most commonly used for mounting either BUIS (Back Up Iron Sights) or small red dot sights off at an angle, preserving the line of sight for your primary aiming device e.g. magnified scope. However on guns with limited rail space, some use the mount for adding a flashlight or laser etc.