Yellow book is commonly accepted internationally, and I would like to keep all my vaccine records in one place to monitor and have a record for my own use and my family doctor. It also eliminates having to print out hard copies from various providers in different format or searching for screen shots on my phone. I usually ask vaccine providers to sign the book and no one has any issues or raise any questions (both in China and US). Just had my Shingles vaccines at CVS before I left the US, and it is in my Yellow book and I can keep track when I need to do my second shot.

It is small in the same size of a passport to put into my passport pouch when I travel.