How do I define the stress field in RS2?

In the Field Stress dialog, the following parameters are used to define a constant field stress.  Sigma 1 is the major in-plane principal field stress, and Sigma 3 is the minor in-plane principal field stress.  Sigma Z is the out-of-plane field stress.  The Angle is measured between the positive x-axis and the direction of Sigma 1.  The Angle is measured in degrees, and the counter-clockwise direction is defined as positive.

RS2 Field Stress Properties dialog

Note that RS2 is plane strain.  Inherent to the plane strain formulation, the out of plane stress is a principal stress and therefore normal to the plane of analysis.  When you transform the 3D stress field to be in line with your plane of analysis, you'll probably find that there is shear stress out of plane.  This shear stress is generally ignored, and the out of plane stress is used as if it was a principal stress.  Whatever you do, you end up making a simplification assumption to adhere to the plane strain formulation.

RS2 has a Stress Transform tool to assist with stress transformation.

Stress Transform dialog