It's pretty clear (albeit all from the experience of those on here) and has been written about several times:

1) Revenue tickets before award tickets; within that:
2) By status tier (SEs before E75s, then E50s, etc); within each tier:
3) By fare class (higher fare class goes first)

So all SEs will be cleared before they go to E75s. SEs will be cleared in order of the fare class they purchased (Y, then B, then M, etc). Once all SEs are accommodated, they'll start the E75s, and again clear them in order of fare class. Then they do the E50s, etc, etc. Only after all revenue tickets have been dealt with will they consider SEs on award tickets.

The only complicating factor involves itineraries that start somewhere other than the point of departure of the flight we're considering. There's some element whereby people can be upgraded based on having started their journey earlier, but it's unclear how that works.