c1d6df67_C18s_Are_the_Same_Aren_t_They___2
[0] S [2] 
Steric or shape effect. The ability of the stationary phase to discriminate between molecules of similar size / LogP, but with a different 3D shape. This can have a significant effect on selectivity. In some cases, this parameter can be exploited to allow resolution of isomers.
All HPLC C18s Are the Same Aren’t They?
2