About the script directive pose_trans

Hello, I was wondering about the pose_trans script directive.
Because I read a related article on your technical support website. (Universal Robots - URScript: Move with respect to a custom feature/frame)
As for the example under Pose Transformation, my personal understanding is that pose_wrt_tool and pose_wrt_feature used in the article are keywords rather than user-defined variables. The reference frame of these two variables has been specified when assigning values. So you can complete the exercise effect.
Is my understanding wrong? Because I just can’t find any other way to explain it.
However, these two keywords could not be found in the manual, so I would like to ask you some questions on what makes the pose value obtained by pose_trans conform to the expected effect of exercise.