GenServer.stop

You're seeing just the function stop, go back to GenServer module for more information.
Link to this function

stop(server, reason \\ :normal, timeout \\ :infinity)

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Specs

stop(server(), reason :: term(), timeout()) :: :ok

Synchronously stops the server with the given reason.

The terminate/2 callback of the given server will be invoked before exiting. This function returns :ok if the server terminates with the given reason; if it terminates with another reason, the call exits.

This function keeps OTP semantics regarding error reporting. If the reason is any other than :normal, :shutdown or {:shutdown, _}, an error report is logged.