First paragraph talks about build. The build by default happens in sandbox, with many things disabled to force reproducibility. It sometimes makes things annoyi

First paragraph talks about build. The build by default happens in sandbox, with many things disabled to force reproducibility. It sometimes makes things annoying, for example there was one Ruby package that was downloading a file during build, so whomever was making a derivation needed to download it in advance) The second paragraph talks about fetching sources which nix does before build starts, each source is required to be provided with hash, this ensures that it is an identical file that author of the derivation used.