Application.start_phase

You're seeing just the callback start_phase, go back to Application module for more information.
Link to this callback

start_phase(phase, start_type, phase_args)

View Source (optional)

Specs

start_phase(phase :: term(), start_type(), phase_args :: term()) ::
  :ok | {:error, reason :: term()}

Starts an application in synchronous phases.

This function is called after start/2 finishes but before Application.start/2 returns. It will be called once for every start phase defined in the application's (and any included applications') specification, in the order they are listed in.