Welcome to the FAQ website covering document topics in Canada.
I live on a farm with no mail delivery to our house. Consequently, we have a PO Box in a nearby town. My mailing address is therefore a PO Box. For the current address requested on the passport form, do I use the post office box # and town, or my municipal address which has a site number and a range road number, but no postal code as we do not receive mail here. The municipal address is used for dispatching emergency vehicles and such.
Passport Canada needs your mailing address to get the passport to you, but they also want your physical address. There is space for both on the form.