I'd like to know too. I think the setting is a synthesis of NVRAM (that is, the setting saved by the shutdown hooks installed when you install clover to a drive, visible in Terminal using the 'nvram -p' command, efi-boot-device setting, which is set in the GUI by the Startup Disk System Preference), and the config.plist setting Boot -> Default Volume setting. However, on my 10.13 upgraded from 10.12, nothing I do to either of these settings will stop Clover from picking the High Sierra disk to startup from. I look in the boot log and I see it reads the efi-boot-device from nvram as the old USB 'install macOS High Sierra Beta' volume, it doesn't find that, and somehow lands on the High Sierra drive. I've tried removing the key from nvram, I've tried manually rebuilding nvram using the shutdown hook command after changing the startup disk, all to no avail. Maybe I'm stuck in some weird upgrade twilight where it's executing the old rc.shutdown hook and the new shutdown hook, or something. I don't know.