From REDS Racing:
REDS Racing expands their manifold range by adding a new Long manifold to the current Medium, Short and Extra Short manifolds. Each manifold has been designed for different track conditions and to optimize performance.
L (Long) NEW manifold allows you to increase performance at low and medium RPM for a better feeling. It is recommended for high grip tracks, top speed will be lower. We advise to use this manifold with REDS 2104 pipe.
M (Medium) is recommended for medium grip tracks where more bottom torque is required, top speed will be slightly lower.
S (Short) is recommended for medium or low grip tracks; it is a good compromise between torque and top speed.
XS (Extra Short) is recommended for low grip traction to achieve the highest top speed but lower bottom torque.
Learn more about the whole manifold range here: https://www.reds-racing.com/accessories-off