When you purchase software through the App Store on your Mac it remembers. The software is purchased under an Apple ID. Then, after you purchase the software it is downloaded and installed on your computer. There is a way to install this software on another Mac if you would like to do so.
There is one catch. All these software purchases and installs through the App Store are tied to one Apple ID. So, both Macs have to be logged into he App Store on the same Apple ID. If they are, and you wish to install a purchased software on another one of your Macs (in my case a 2011 MacBook Air) just open the App Store and click on the “Purchases” icon like so:
Not a big epiphany for sure, but I just wanted to remind people it is easy to do this. I did it on my wife’s MacBook Air a couple of days ago, piece of cake.