Originally Posted by BananaTugger

M30S, T-700 and T-800 are simply manufacturer designations for that particular weight and modulus of carbon fibre.

What is more important to the use of carbon fiber in bicycles are the exact values of its weight, strength and modulus.

The Modulus of Elasticity (I'll just call it modulus), is how much the material stretches when stressed. This value is directly related to how stiff the material is. A high modulus material like Toray's M40 carbon fiber is very elastic and resilient compared to Toray's M30S carbon fiber, which is much stiffer and stronger in tension and torsion. Because the M30S fibers are stiffer, they are more likely to break when they are stressed to much. The M40 fibers will continue to stretch while not breaking when they reach their threshold. It is possible to deform high modulus fibres however.

Modulus is measured in GPa (Gigapascals). High Modulus materials are in the 250-350 GPa range, while low modulus/high strength materials are in the 100-250 GPa range.

Using a high modulus material reinforced by a low modulus/high strength material in certain areas is how most frames are constructed.

Using high modulus fibres for the seatstays will allow them to bend and give a little when they are compressed. Conversely, using low modulus/high strength fibers in an area that sees a lot of torsion and tension, like the bottom bracket or headtube, will make that area of the frame stiffer than the high modulus material would.