What are side rails exactly?

[GA_Rivian]

Can someone explain to me exactly what the side rails on the R1T are?
Just got the note saying if I don't order the manual tonneau cover, I don't get the side rails.
Are they something added onto / cut into the top of the bed sidewalls?
I had assumed they were a permanent part of the bed, but apparently not.
If they were, I could make my own manual cover, although I probably wouldn't. But an aftermarket provider might.

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[Prime]

Yellow circle = side rails.. If you dont order with manual cover it will not have the rails and you will not be able to add them later.