I am building a plan and using the Gantt chart. I have a plan item X with several execution items as children (A, B, C). The plan item (X) has a sibling execution item (Y). I would like the sibling execution item (Y) to treat the plan item (X) as a predecessor such that the execution item (Y) cannot begin until the plan item (X) is resolved. If I set the execution item's (Y) predecessor to the last child of the plan item (X/C), this works just fine. The execution item (Y) does not start in the Gantt chart until after item C is finished. However, if I set the predecessor of execution item Y to plan item X instead, then the execution item (Y) is allowed to start immediately (concurrently with the plan item (X)).

It appears that there is a rule that the Gantt chart doesn't work unless predecessor/successor is execution item to execution item (and presumably plan item to plan item). Is this correct or am I missing something?

Is there a reason why it is implemented this way?