In simple terms, a Stock ROM (Read-Only Memory) is the operating system that comes pre-installed on your phone by the manufacturer and carrier. The "Official" tag signifies that the software is signed by Sony and Docomo, ensuring:
For those looking to breathe life into a SO-01J today, the Stock ROM remains the most stable, "plug-and-play" option—provided you can tolerate the icons for apps you’ll never use. It is the operating system that proves that in Japan, the carrier is just as important as the manufacturer. Official Sony Xperia XZ SO-01J -Docomo Japan- Stock Rom