Last latter/number of version number indicate SPIFFS file system version. If the first (upper-case) letter has changed, then this version is incompabible with prevision versions and you have to flash the full image. If the second part (number) has changed, then this version has some changes (e.g. internal web page layout, LCD/TFT display layout) in the file system which you will not get with a code-only (OTA or flashing update.bin) update, but it should not break anything.