Work permet rules—different jobs/employers/locations?

A few years back, the law regarding work permits was modified (more specifically, the "Emergency Decree on Non-Thais’ Working Management," sections 70, 71 and 73). Before, it was only possible to work for one employer at one location, but now it's different.

What are the current rules for work permits? Is it possible to work for multiple employers, at different locations, or different jobs? Or work for yourself—a second job, working for yourself, while still doing the first one?

(For the more technically inclined, the new official translated document is Foreigners's Working Management Emergency Decree (No. 2))