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

handle_deallocate(query, cursor, opts, state)

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handle_deallocate(query(), cursor(), opts :: Keyword.t(), state :: any()) ::
  {:ok, result(), new_state :: any()}
  | {:error | :disconnect, Exception.t(), new_state :: any()}

Deallocate a cursor declared by handle_declare/4 with the database. Return {:ok, result, state} on success and to continue, {:error, exception, state} to return an error and continue, or {:disconnect, exception, state} to return an error and disconnect.

This callback is called in the client process.