What's the difference between `def` and `defp`

From Elixir’s documentation on functions within modules:

Inside a module, we can define functions with def/2 and private functions with defp/2. A function defined with def/2 can be invoked from other modules while a private function can only be invoked locally.

So defp defines a private function.

So my question is how do I know when to use defp and when to use def when defining a function inside a controller in the Phoenix Framework.

def functions of a module can be called from other modules, whereas defp functions are private, or not callable from other modules. How do you know when to use def and when to use defp? It depends on what other modules may or may not need to know about. A common design pattern is for a module to provide a parent def function that wraps all the behavior of its defpfunctions:

defmodule MyModule do

  def function do
    # call all the defp functions below to do something
  end

  defp function2 do
    # do something that no other module cares about or needs to know about
  end

  defp function3 do
    # do something that no other module cares about or needs to know about
  end

  defp function4 do
    # do something that no other module cares about or needs to know about
  end
end

Here is an example of this with a parser for SEC filings: SEC Company Filings Parser. The main def method wraps all the private functions which no other module really needs to know about.