Rooms are virtually never square, level, or plumb. Ceilings tend to sag toward the middle of their rooms; floors usually do the same. Plaster walls are rarely flat; drywall builds up at interior and exterior corners. You get the picture.
Designing built-ins is an art that takes contextual imperfections into account and makes dealing with them as easy as possible. A common way of handling these points of intersection between a cabinet and its surroundings is to cover them up with trim: Think cove moulding, quarter-round, shoe moulding, crown. Applied kicks are another example. But some historical styles call for minimal applied trim. In these cases, it’s customary to handle the gaps between built-ins and their settings by scribing, a method of marking a built-in precisely so that it can be trimmed to conform to its context.
In a previous post I described a method for scribing applied drawer faces to fit their face frames.