From the Nikon SB-800 manual:
"Press the MODE button to display TTL on the LCD panel, and Starndard TTL flash is performed. The main subject is correctly exposed regardless of the background brightness. This is useful when you want to highlight the main subject."
What is the main subject? Is it the entity closest to the lense? Is it the area highlighted by the AF bracket? Is it the centerweight of the frame? Is it any entity on the focal plane?