It may refer to the criticality of line to determine the level of stress analysis to be undertaken.
My guess, SC is the term of "Super Critical", C is "Critical", M is "Moderate". For I, I haven't idea about that.
For SC and C, the system analyzed by computer program e.g. CAESAR II and FEA may also be required.
For M,formal analysis will in general done by computer program. However, it may also be done by approximated hand calculation.