Ed is correct, however decalage refers to the main wing only. If the stab incidence is 1-degree and the wing incidence is 1.5-degrees the decalage is .5 degree. The reason this is important is adjusting the stab incidence has the effect of changing elevator trim, adjusting the decalage effects the flight characteristics of the plane. Planes fly based on the lift generated by the angle of attack of the wing at a given airspeed. Since this is the constant changing the main wing decalage will change the thrust line, fuselage angle of attack, inverted flight performance and trim as well as a number of others. :^)