Napoleon contential furnaces

[Mmtc79]

Questions on page 22 of install manual states
If installed in cold climate venting must be periscope with 18 inches from bottom of fresh air to exhaust ?
Key word is must is that to say these units can't be vented with concentric vent kit sidewall ? Or the 636 flush mount kit ? So many questions left unexplained ?
For example who or what determines if your install area is a extreme cold zone ?
-30 degrees Celsius for 48hrs ? Is that definition in the code book ? No
Is that with wind chill or not ? I feel it's such a grey area I thought I would post on here to see what others are doing
Obviously if you can get the 18 inches no problem my question is if you can't then what ?