Someone on this very forum has posted up the map from the old AEI renix unit. Basically it has a map which is used when there is no vacuum in the manifold (positive pressure) When the throttle is closed or just open (cruising conditions) and there is vacuum in the manifold it switches to another map, which is the same as the first map, but plus 20 degrees advance.
This is because the engines requirement for IGN advance is very different depending on what it is doing. It probably only wants 24 degrees or so when the throttle is on the floor, but when you lift to cruise, you in effect have a low compression engine which will take a silly amount of advance, as it has no BMEP. this also has the effect of minimising any energy in the exhaust gas which puts the turbo into cruise mode also. giving reasonable fuel economy.
An atmo cruising at 4000 rpm is probably going to have about 26 degrees of advance on and if the vac unit is connected it will be adding around 6-9 degrees onto that. When the throttle is opened the vac unit drops the timing back so it doesn't detonate as the BMEP rises.
Turbo engine is no different, but will take a much larger swing as it is nowhere near optimised off boost. Cruising at 4000 rpm it will probably run 50 degrees advance as it is already a low comp motor, plus the throttle is shut meaning its running much less than 7:1 so you need to light it very early for the slow burn time.
This is the most crucial part of any engines set up and is dependant on the actual running conditions at the time. most people get lost in the ultimate power game on turbo engines, but getting the low boost/ cruising map right gives a much nicer engine to live with in reality. Getting the transition between the two correct results in less wreckage.........