What is "documentation of your distribution and/or your additional material"?

[Tragetaschen]

The To-do lists contain the phrase "Let the documentation of your distribution and/or your additional material also…" on several occasions (for example in Apache-2.0-C2, Apache-2.0-C3, where it's "voluntary" or BSD[23]-C3 where it's "mandatory"). I was not able to deduce what I should do for license compliance.

Could you add an explanatory section regarding what constitutes "documentation of your distribution" or "additional material"?