Clarify fixed output derivation steps

[abacef]

In the section where it says For fixed-output derivations, run the build four times

1. Why is four times the correct number of builds to reproduce a fixed output derivation
2. What is a fixed output derivation? And how can I tell if I have one? Is there such thing as a variable output derivation?
3. How does one run these four commands

[raboof]

What is a fixed output derivation? And how can I tell if I have one? Is there such thing as a variable output derivation?

'Normal' derivations are identified by their inputs. Fixed-output derivations are described at https://nix.dev/manual/nix/latest/glossary.html?highlight=fixed#gloss-fixed-output-derivation . These are the derivations where you specify the 'hash' of the result in advance, such as fetchers.

Why is four times the correct number of builds to reproduce a fixed output derivation

To be honest looking at that documentation again I'm not quite sure that it is correct, and that building it with --check/--rebuild wouldn't work fine. This would be something to double-check.