Flow Simulation - Flow Plot - "Opening"

Hello

In the "Flow Plotting" tool, I can't understand where the heat flow comes from towards what is called "opening" (see attached image): it is not a boundary condition, nor a transfer to the computing domain, to another fluid, to a solid or other... The color of the arrow does not correspond to any of the categories in the diagram.

Can I be enlightened?

Thank you in advance.


capture_2.png