I searched for some information about it, but it wasn’t very detailed. so,
Q1: Which countries print barcodes on mail？
Q2: What mail are they used for, imports, exports, or both domestic and overseas?
Q3: Where to check their standard requirements? Or how much space should I keep underneath the card when mailing a postcard to these countries？
Plus 1, Q4: What type of barcode are used for these?
I ask this question because I know that in my country, China Post uses barcode stickers on domestic mail and international registered mail, and I know the type of codes so that when I need to mail in bulk, I can ask CP for the codes in advance and print it myself, although this demand is rare.
This won’t answer all of your questions, but the barcode you’ve highlighted is used by one of USPS’s sorting machines as a unique ID for when a mailpiece being sorted needs to be held while its address is determined, looked up, or confirmed (likely in this case due to being a non-Canadian international address; seeMultiline optical-character reader - Wikipedia).
In Germany all mail (which is sorted automatically and) with destination in Germany is coded in orange, no matter if it was sent nationally or internationally. It will be printed over any writing on the bottom, but since it is orange, it’s not a problem to read what is written beneath it. Here’s a a video about it (only in German, but with auto-translated subtitles):
The code has information about the target address: street, number, zip code
Pretty much all mail that goes through the US gets a barcode, usually on the address side, sometimes on the reverse. I believe the barcode is unique to the mailing address, for the sorting machines. Here’s a graphic from a 3rd party website noting the space needed for a barcode.
I believe the stickers are theoretically for surfaces where the barcode won’t otherwise be visible (but the machines aren’t great at figuring that out). Personally, I like the stickers because they’re so easy to remove.
