Documentary

I believe that is very hard to draw the line between any kind of photography and documentary photography. Why? Because all images which was taken years ago, and also images which only going to be taken in the future is already some kind of documentary. A documentary of moment of time, a documentary of event or thing what happened only at that time, only once in the life. I am thinking about this project and finding really hard to decide what kind of images to use. Maybe, my understanding about documentary photography is wrong, and images I likely use for this project is not a documentary images.