As I understand it if you buy a piece of software from MAS or the Applke Store that same software may be installed on up to five "authorized" devices. From experience with applications purchased on MAS you can install or re-install an unlimited number of times on any given "authorized" devoice.
I discovered this in the very early days of the App Store when I was having difficulty getting some MAS apps to installl and run properly. There is only one way to enable a reinsall which is to uninstall the app, but once it is uninstalled the App Store will happily install any app you have paid for.
