Is the phrase “concave portions” part of the definition, or is it an example? I’m sure robots can control MOGOs in ways other than “concave portions”.
A robot can control mogos in other ways (clamps, lifts) but as far as pushing goes, according to the rules a flat surface would not count as possession. (Example you can touch/push a mogo with the side of your robot but it does not count as a possession.) The reason this rule is I place is to prevent teams just touching 4 mogos in the corner and calling it possession. Do I think that the rule is easly justified.
The phrase “a Robot using concave portions to control the movement of Mobile Goals is considered Possession.” does not mean that other things can’t also be considered possession, provided they meet the definition above.
For example. grabbing a mobile goal branch with a claw and moving it around the field is not “controlling the movement of the goal using concave portions of the robot”, but is definitely possession of that goal.
I think the concave portion is to defend against the argument “Team X with a flat surface is pushing and team Y with a concave robot is just pushing so there is no difference.”
This argument has been around for years even though concave meets the turning definition the GDC wanted to qualify the robot surface argument with an example.