There are two edges and hence two edge angles. So you have a base edge and a side edge.
I know very little but from what I understand, you tune your BASE edge angle to less than 90 degrees, say to 89 degrees to avoid catching an edge on features like boxes and rails. Some may suggest a more drastic change in the base edge angle for this purpose.
Then if your edge hold is sub-par, say in icy hills, you could consider tuning your SIDE edge angle from 90 degrees to 89 degrees to see if there is any improvement. Again some may suggest a more drastic change in SIDE edge angle, but for starters 89 is commonly quoted. Obviously changing the SIDE edge angle is only just one of the several options you could go into to alter your edge hold.
Also everything depends on what your factory default angles are to begin with. Go with how you feel as you change things incrementally. Basically you can go from 90 to 89 to 88, but it's quite impossible going back to 90.
Detuning is usually applied to the nose and tail plus an inch's length into the effective edge (past the "contact points" at either ends). The purpose is also to further reduce the likelihood of catching an edge again. No angles are quoted in detuning as all you do is to render a sharp edge to one which is more rounded, with, for example a panzar file in a gentle manner.
Tuning (the part of the board excluding the nose and tail) the BASE edge and SIDE edge changes their respective angles. Tuning does not effectively blunts the edges. Sharpness is still maintained.
Detuning (nose and tail) blunts and removes the sharpness of the edge.
I would like to learn more too so please correct me if my concepts are wrong.