apps 
The app and apps options allow you to set apps.${system} outputs.
apps is an attribute set of apps or a function that takes packages and returns an attribute set of apps. If the app value is a function, it is passed packages. If the app value or function result is a string, it is converted to an app.
app sets apps.default.
Usage 
For example:
nix
{
  inputs.conflake.url = "github:ratson/conflake";
  outputs = { conflake, ... }:
    conflake ./. {
      apps = {
        emacs = pkgs: "${pkgs.emacs}/bin/emacs";
        bash = pkgs: { type = "app"; program = "${pkgs.bash}/bin/bash"; };
      };
    };
}Alternatively, the above can be written as:
nix
{
  inputs.conflake.url = "github:ratson/conflake";
  outputs = { conflake, ... }:
    conflake ./. {
      apps = { emacs, bash, ... }: {
        emacs = "${emacs}/bin/emacs";
        bash = { type = "app"; program = "${bash}/bin/bash"; };
      };
    };
}