What is the round dial by the bag symbols for on the electrolux epic series 6500 sr for

SOURCE: Repair manual for Electrolux EPIC 6500 SR

I don't have a manual for these but the trickiest thing about these units is taking them apart. If you have an Aerus/Electrolux dealer near you, it's not a bad investment to have them install the motor for you. You would probably receive a warranty with the work, and the things a long time. Great return on investment. But if you are determined to do it yourself, open the front of the canister and remove the bag. As you are looking at the front of the unit, there are two vinyl coverd strips that run the length of the unit. Pull forward on these strips and then pull out so that the vinyl seperates from the plastic underneath. Now grab the plastic strip with a pair of needle nose pliers and pull towards the front of the canister. These can be tough if the machine has never been opened up before, but pull as hard as you need to, to slide the strip out. Once both strips are out, turn the machine over, pull out on the sides of the bottom of the unit, and lift up. The bottom should come of relatively easily, and eveything will be exposed. Good Luck!!

SOURCE: i have the electrolux epic 6500 sr. when i

As hooton8 stated i have the electrolux epic 6500 sr. as well and when i attempt to vacuum i turn it on and it turns off after about 3 seconds. new bag nothing stuck in tube. I turned dial to the right all the way. Works when hose is not connected..hose is clear.

SOURCE: Our Electrolux Epic Series 6500

This is most probably due to a worn out or clogged Control Valve Assembly.
Detach the hose and see if it runs. If it does, place hand over hole. If it shuts off, CVA is probably the cause. A clogged CVA is generally due to using generic/made-to-fit bags. Genuine bags have "Home Solutions" printed on them.
You can try turning the dial all the way to the large dot, then try to run the vacuum.
If it is the CVA, you'll need to take it to an authorized Aerus Electrolux store and have their service tech repair or replace the part. I recommend a service and clean of the vacuum at the same time. This will extend the life of the vacuum.

SOURCE: what bags are used for electrolux epic series 6500

Those would be Style C vacuum bags.