A scalar summation just sums the magnitudes of the two stresses, which is what the Code requires. (If you change the summation to Algebraic, then the sum of the displacements would be used to recomputed forces/moments, and then stresses. This is not what the Code asks for.)
The SRSS method determines the vector sum of the components.
When you're in the Load Case Editor, click in the "summation type" column, then click "F1" and review the help text. There are numerical examples here that will illustrate the different methods.
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Regards,
Richard Ay - Consultant