The allowable stresses are obtained from ASME Section VIII-2 Part 5, Design by Analysis.
The code made significant changes in 2007, and then made small changes for version 2013.
The user can select the applicable code from Options/ASME Rules:
SPL is the primary stress allowable, the highest of Sy and 1.5S.
SPS is the secondary stress allowable, the highest of 2Sy and 3S, average for hot and cold for both.
Sa is the fatigue stress allowable, this depends on the type of curve selected in the ASME rules options.
4S is defined in paragraph 5.3.2, this is 4 times the hot allowable stress.
User is defined as either SPL or SPS depending on the category they fall into. Some bending stresses are primary, some are secondary. These values depend on the user entries for allowable stress and yield strength in the materials window.
Here is a small sum up from the main changes.
All these definitions are defined in ASME Section VIII-2 Part 5 as mentioned earlier.
Paulin Research Group