Picture is not coming thru. However, if the hole you're speaking of is approximately 1/8" in diameter, possibly slightly smaller... that is one if the water drains built in to the unit. It's purpose is to allow water to drain out of the unit in areas where freezing temperatures take place (eliminates cracking).
If you're speaking of a larger hole (approx 3/8" or so) that may exist in a 5" extension housing... the same holds true (drain hole).