The Administrative Manual describes the qualification procedure for districts. Here is the
2015 manual I expect the 2106 will be similar. Obviously we will find out when they issue the 2016 rules.
A few of the bigger differences
District events have fewer teams and more seeding matches per team.
You get an eight (actually six) hour unbag window to work on you robot outside of the event. This can be divided into short periods. This is to compensate for the lost day compared to a regional. The same fabricated parts allowance applies to the district event and unbag window as a regional
You earn points at two district events to qualify you for your district championships (DC). The point break down is in the administration manual. Event winners are also invited to DC. You do not qualify to go directly to worlds. The same regional event awards that get you to worlds will get your there at a district championships. After that it based on points earned at the championship + district events and slots available to determine if you go to worlds.