What are "outer boundaries" and "internal boundaries"?

Hello.

When I read the Corrosion Module User's Guide, I could not figure out the meaning of "outer boundary" and "internal boundary" exactly.
So I want to know the exact meanings of them.

In the "Electrode Surface" section at p.79 of the CorrosionModuleUsersGuide.pdf(ver5.2), it is said:
"This node can only be applied on outer boundaries to electrolyte domains. For internal boundaries between electrolyte and electrode domains, use the Internal Electrode Surface node."

This means that there are two nodes for setting the electrochemical reaction at the electrode surface and I should choose one of them case by case.

Please tell me what are outer boundary and internal boundary.
And I also want to know the example using Electrode Surfce node or Internal Electrode Surface node (if possible).