Erik Selin writes:
Physically based rendering in Blender has been a guesswork for some time. With 2.79 however comes the principled shader. It will help you to create accurate blender materials for cycles. However, there still seems to be some confusion on how it works. Let’s get a closer look at it and nail Physically based rendering once and for all.
Physically based rendering or PBR for short is a way for ray traced render engines such as cycles to accurately describe a material. It lets the artist focus on the artistic side to a larger extent and leaves out how to deal with more technical issues. This information is a set of guidelines and is not written in stone. It is here for you to get an idea of how PBR works with the principled shader. Once you understand it you can break and bend the rules to your will, but a good foundation to start from is better than to dive in without knowing how the shader reacts to different settings or texture map inputs.