What does it mean if the critical SRF determined by RS2 is less than 1?

For one stage models, RS2 accurately computes models with SRF or FS less than 1.  For multi-stage models, if and only if all stages leading up to the last stage converge is the solution accurate.  RS2 uses the stress state from the end of the second last stage as the initial stress state for the SSR analysis.  So if it is invalid (due to non-convergence) the SSR analysis will not be correct.  There are numerous examples of one stage models with SRF<1 in the Slope Stability Verification Manuals in the RS2 Online Help.