On system start up I currently see Linux 4.0.0-rc6yy and 4.0.0-rc6yy.old from the bootloader menu. I'm not certain where they came from. I suspect "yy" is arbitrary but can someone explain the ".old" suffix?

Also can someone explain what CONFIG_LOCALVERSION and CONFIG_LOCALVERSION_AUTO is from .config? I've looked them up but am still unclear about their use. Many thanks.