Why is \r a newline for Vim?

From question How to replace a character for a newline in Vim?. You have to use \r when replacing text for a newline, like this

:%s/%/\r/g

But when replacing end of lines and newlines for a character, you can do it like:

:%s/\n/%/g 

What section of the manual documents these behaviors, and what’s the reasoning behind them?