From the introduction:
As the Artifact Retrieval Corps badge indicates, the three most important attributes you bring to the game are Body, Mind, and Spirit. It is from these that we are able to calibrate your personal reader. These also give a good sense of just what you are going to be able pull off in the field.
Body and Mind are pretty self-explanatory. After encountering a vast number of beings over the years, the Interstellar Congress has a pretty grip of what sentients are capable of and have set a five-point scale to describe them. The higher your rating, the better you compare to the rest of the sentients in the galaxy.
Spirit is a little more nebulous. I mean, it works the same way, with the Interstellar Congress' scale and all, but I've got a lot harder time wrapping my mind around what it is. Body is physical stuff. Mind is mental stuff. Spirit, well, I guess that's pretty much everything else. If you want a better description, go find a Murpei.