path_constraints vs trajectory_constraints in motion_plan_request

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Hi, As I know, you may need to add constraint to the path which for example an arm wants to move along an area to keep a glass of water in a specific direction. I see there are two properties in motion_plan_request in MoveIt which one of them is path_constraint and the other is trajectory_constraint. In my mind the name and the type of them seem to be the same in concept. I want you to explain me what is the difference between these two. I also cannot find an example which uses trajectory_constraints for a motion planning test.

Best.