ABC/R (Augmented Bolt Catch/Release) with its angled and larger top paddle and lower paddle with increased surface area, is purpose designed for use with AR15/M16 receivers equipped with ambidextrous magazine releases. Specifically, it is designed to address the difficulty with accessing the factory bolt catch's lower paddle with an ambidextrous magazine catch, such as Norgon's Ambi-Catch, on the receiver. Many of the ambidextrous mag release's lower paddle is on the same vertical plane as the factory bolt catch lower paddle, making identification difficult by feel, especially with a gloved hand.
FB (forward biased) is for right hand users that wrap their left hand around the magazine well, and use their left thumb to press the bolt catch's upper paddle. The FB version has the upper paddle shifted to the muzzle end to facilitate this reloading method.
