Source and decoding
אור לארבעה עשר בודקין את החמץ לאור הנר. כל מקום שאין מכניסין בו חמץ, אין צריך בדיקה
On the 'or' of the fourteenth one searches for chametz by the light of a candle; any place where chametz is not brought needs no search.
The Mishnah opens the tractate with the time and the means of the search. The Gemara at once asks what 'or' means: Rav Huna says 'nagehei' and Rav Yehuda says 'layla'. The Mishnah also limits the duty to places where chametz is brought. So the discussion is set up around time, instrument and scope.