The computer advances or retards the timing due to various conditions it picks up on through sensors. Let's say you have your timing set at 0 degrees, the computer (if it deems fit) can advance it. Or you can set the initial timing and it could see that it's too far advanced and it can retard the timing.
According to my 5.0 Handbook, if you set your initial timing to 14 degrees, the computer will produce approximately 38 degrees of total ignition advance. So that's 24 degrees that the computer can play with.