What do Outputs: out or dev mean in NixOS Search
Packages that have those listed are split into multiple /nix/store paths. That way, Nix-built packages do not have to depend on entire dependency packages, only what is actually needed. For example, you do not usually need library header files or other development files for running an already built program. And when those dev outputs are not part of the runtime closure, Nix will not need to pull them from the binary cache along with the program.