The format of Android build numbers has been the same for years, but it’s changing in Android 8.0. The first non-preview version of Oreo just rolled out with the new format, which is a big departure. Rather than being three letters followed by three other characters, it’s split into three blocks of numbers and letters separated by periods. It looks confusing at first, but you actually get more information from it.
[Update: AP build number calculator now fixed] Google changed the build number format in Oreo—here’s how to read it was written by the awesome team at Android Police.