As a follow-up to the post Feature Line Styles Control What?:
‘Featureline Priority’ is most likely a feature – no pun intended – in Civil 3D that you have overlooked or didn’t even know about. As you do know, if Featurelines are in the same ‘Site’ they interact with each other and the last one edited wins.
For example:
If you have two Featurelines that cross each other and are in the same ‘Site’, Civil 3D will force them to be at the same elevation where they cross. This is a good thing since they need to be to build an accurate surface (review image below).


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