I am trying to figure out the difference between various "Principal Stress" components for  a simple plane-stress problem in Abaqus CAE.  There are a total of 8 of these with no explanation provided anywhere in the manual, except for the generic/cryptic tensor depictions. Could someone please provide a succinct explanation of what these things are supposed to be?

As a test, I ran a simple plane-stress problem, and extracted S11, S22, S12, and decided to find the principal stresses using the Mohr's circle equations.  With output of S11 = 58, S22= - 8.85, and S12 = -1 (all probed at the same location), my calculation showed max principal stress (Sigma1) to be about 55.8, and none of the Abaqus "principal" values were close enough (the closest was 58).  Clearly I am missing something here. Any clarification would be greatly appreciated.