Dear Krishna.
Read article, Tip paragraph: Derivation type: Move
TIp: Pay attention to Step "Standard". It is without condition. It means it will always be executed - our transaction will reiceive Account Assignment Reference - REF_OTC. But ii is wrong. It is because our new step is below "Standard" setp and have target field attribute "Do not overwrite with new values if field already filled". Don't change this parameter, but move your derivation step above.