help understanding how to select equipment

I am having a difficult time understanding how to guide the mechanical designer on specifying compliant equipment. Are there any best practice advice or rule of thumb I could be aware of, or questions to ask, when approaching this credit at the beginning of the project?  On most of my projects I have not had any issues achieving this credit, but on a recent project we didn't get this credit. Now, I am questioning my approach and it would helpful to know what type of systems wouldn't be acceptable at first glance, before running the calculator. Thank you.