What are these A2 and A3 stickers? I just read the manga travel by Yuichi Yokoyama.

What is the meaning of the A2 and A3 stickers seen on packages and documents carried by strangers when the 3 main characters walk inside the train? A3 is on small tubes, and A2 is on a folder and an A4-sized box, so I guess it is not ISO 216 (paper size) though.

This manga contains zero dialogs and nearly zero text of any kind, except for A3 and A2, which are shown prominently.

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Left: A3 stickers on page 30 | Right: A2 sticker on page 31
While it's true that this manga lacks any dialogs, there are actually 2 pages at the end (pg. 196-197) that describe the whole scenes from the beginning until the end.
For page 30-31,


30-31 鞄の中にファクシミリ用感熱紙、コピー用紙、ファイルブックなどの文具又は書類が覗いている。
30-31 Thermal papers for facsimile, copy paper, filing book and other stationaries or documents are peeking through the bag.

Researching those things returned these images:
 
Thermal papers for facsimile by Fujifilm | Copy papers by Super Echo
Thus, I believe spiel's comment was actually right: they are standard paper size labels. The A3 thermal papers are rolled into a cylinder, and the A2 copy papers are showing their shorter side. (Which means that the filing book is also A2-size, which also exists)