[84LX89GT]

Base/Initial timing is what you set your vehicle at without any kind of vacuum, mechanical (both for carbureted vehicles) or electronic controlled advance. Your engine will adjust timing from that point, or in some race cars the base timing is advanced to full advance because you don't need an advance curve in a certain rpm range.
Total timing is the base(initial timing), plus any kind of ignition advance provided by a computer, a vacuum diaphragm or by advance springs. Total timing can be the same as initial timing if there is no additional timing advance added. alot of vehicles run from 28* to 40* before top dead center total advance because since the engine may be at a higher rpm, it still takes a certain amount of time for the air/fuel to burn. Combustion chamber design, engine rpm, compression ratio as well as camshaft design has alot to do with how much timing you need at the top end of the power band.