[boost] Understanding Jam Files

I have been working on boost for about a month now, but I still don't fully understand boost's build configuration very well. More specifically, I do not understand where the properties array gets filled in and where that data comes from. I saw the property.jam file which seems to refine the list of properties and take away unmet conditions, and bind dependencies. I also saw the property-set.jam file which creates property lists from raw properties, but where do the raw properties come from exactly? Does every property-set created come through this file? It would be very helpful if someone could point me in the right direction. Sincerely, -Axel