Question

As per rails usage:

$ rails new --help
Usage:
  rails new APP_PATH [options]

Options:
  -b, [--builder=BUILDER]        # Path to a application builder (can be a filesystem path or URL)
      [--old-style-hash]         # Force using old style hash (:foo => 'bar') on Ruby >= 1.9
      [--skip-gemfile]           # Don't create a Gemfile

What does this “application builder” refer to, and how does it compare to an application template (-m option)?

Answer 1

Descriptions in API Doc

The application builder allows you to override elements of the
application generator without being forced to reverse the operations
of the default generator.

In a nutshell, you can define your default set of dependencies before creating a new rails app.

Check this post for more info.