Assignment Owner vs. Default Assignment Owner - what's the difference?

I have been told that the default assignment owner and the assignment owner could be two different users. Now it seems that there is only one assignment owner field in the assignment record. So only one person can provide status (or change status - like actual work) for an assignment record.
If that is true, how does the new default assignment owner field fix the "resource is on vacation" problem?

Bottom line:  does the resource in an assignment always have the power to change its status? even if he is not listed as the assignment owner?
Another way to ask the question - is it possible to set up PS2007 such that two users can actually change 'actuals' status of the same assignment record, or is it limited to just one?

Thanks for your help,
Ken