when we talk about roll or pitch coupling with the rudder??

when you apply rudder, for aerobatic models, you only want the rudder to yaw the plane, with no roll or pitch.

some models may pitch either way when rudder is applied, or roll either way, some models roll and pitch which makes knife edge very hard work.

if it is only a little, some people (cheats) mix it out on a program on the transmitter, some do it on the sticks.

a number of things can cause it........

dihedral, centre of gravity, decalage, thrust angle, fuselage shape, i am sure there are lots more