You will mostly be using TTL with the SB-600 and D70, so it's not a major thing.
Sorry, to correct my previous post - SB-600 doesn't have A or AA mode.
And what is A or AA mode ?
Certain speedlights have a non-TTL Automatic flash control mode (A mode), where you set the ISO, zoom and aperture and the flash automatically controls the output using it's own built in light sensor (in contrast to TTL where it is the camera body). AA (Auto Aperture) is a refinement of A mode available with DSLRs where the settings (aperture, zoom etc) are automatically transferred from the body to the flash, but the flash still controls the output.
Some say A mode produces more consistent results than TTL mode in certain situations.