Hi Deb,
the relevance function should be the right place to implement this logic. I do not understand why you need change mode to determine if the freight unit has been planned on the freight booking or not.
I would suggest to check either field PLAN_STATUS of specific stage (STOP_SUCCESSOR node) if needed or PLAN_STATUS_ROOT on ROOT node of freight unit. By checking one of the fields, it should be possible to determine if freight unit is planned and set relevance to TRUE only then.
Best regards, Daniel