its a mark used by the installer to know that the system installed correctly
in fact, "dumped" should be considered a correct installation, but the value is changed to "installed" after the first boot
this ensures that the user "can" boot the installed system, solving problems like a wrongly-installed grub, or drivers, or anything... if user is able to boot at least one time the installed system, then the system is considered correctly installed
so the installer verify first that the "scanned for elive systems" has this value correctly set, if not, it doesn't consider it a sucessfully installed elive (and it wont ask to upgrade it)
this explains every detail: How to update your Elive, the correct way - #9 by yoda
for "keep installed apps", i have added to that link an explanation of how to do it 
this is already included, the user don't need to do anything, but to benefit all the features sometimes you need an upgrade mode (everything is explained on that FAQ)
i3lock-fancy, add a hotkey to it and you are ready 
Going to work on the new "beta download" page in the elive website soon
