MoveIt contact information meaning (FCL library)

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What is the exact meaning of the "Eigen::Vector3d pos" in ContactMap from the FCL library used in MoveIt? In the docs it says the following: "contact position, in world space ". But this still remains quite vague to me. A collision always happens between 2 bodies with a volume, so how can you reduce this collision to a single point in space?

Can someone tell me the exact meaning of this variable? Thanks in advance.