Firebox Door Vent Theory

DaveOmak has posted a lot of info on this, but I haven't found any direct answers to my questions. Digging through old threads on this subject I found info like (I'm paraphrasing): "before I installed an upper vent on my firebox, I had to open the FB door when I added any wood to a fire mid-smoke - otherwise it would produce tons of white smoke. Now I just open the top vent and it burns well."

Is that the main function of the upper vent - to make smoke more blue than white? Or is it used to control temperature more? Or both? Or something else?

Thanks Trucknchuck, Gary S and DaveOmak. I smoked some salmon yesterday and experimented with the vents - trying to see how the two worked for controlling temperature. I think that I can see how the upper one has an effect and allows you to keep the bottom vent open as little as practical to conserve wood. They do seem to work well in tandem. I'll keep playing with them and learning. Thanks.

DaveOmak: One of the reasons I redid my doors and CC top was to seal everything well. I am thrilled at how well everything seems to be sealed. Even when I first start the fire in the smoker and the exhaust isn't warm enough to draw air/smoke and the fire is producing tons of white smoke - almost zero smoke leaks out any of the doors or top. I think that is really going to help me be able to control the fire and temps well. I'm very happy with how it is all performing so far.