A regular shirt virtually forms a right angle between the arm and torso. It’s a slightly boxier construction than contemporary tailored shirts, but does have the advantage of accentuating the upper body by giving it a crisper shoulder-line and fuller torso.
Slim Fit is for leaner guys who are in good shape, and have a V-shaped torso where the chest and shoulder is larger than their waist. We use darts and a higher armhole to create a taper from the chest to the waist that leaves no excess fabric to bunch or billow.
Modern Fit is not too boxy, but it has more room than a Slim Fit.