What is "restricted evaluation mode" in Nix?

Both fetchurl and fetchTarball entries in the Nix manual refer to “restricted evaluation mode”, which is a link pointing to nix.conf - Nix Reference Manual. The only place it is mentioned there is the allowed-urls option, but it doesn’t explain it anywhere. The [ Release 2.0 (2018-02-22) page also mentions “restricted evaluation mode”, but it also doesn’t help much.

Found these 2 issues when googling:

Is this mode still relevant? If yes, where can I read more about it? (Someplace other than the source code, that is.)

Opened What is "restricted evaluation mode"? · Issue #8088 · NixOS/nix · GitHub

@toraritte please open a documentation issue in the Nix repo with that exact text, this is a good task for the docs team to address.