Elon Musk explains it in this answer to Senator Shelby. The whole committe hearing is interesting, but your specific question is answered at 53:02 in the video, which I link here:

https://www.youtube.com/watch?v=O_azyt1JhI0#t=3182

It's poorly-defined, but includes a wide array of reviews, audits, inspections, etc.

See PDF page 17 of http://www.gao.gov/assets/520/511460.pdf