Rev1 = the game with the update pre-installed you can get rev1 from the freeshop, but I think rev1 is bad in my opinion.
Rev0 = the game that starts on v1.0 you need to update it via the eshop or CIA most Mario kart 7 cartridges are rev0 its usually the preinstalled games that are rev1, I have rev0 Eur (my legit cartridge of Mario kart 7 from 2014) and I have a CIA of Mario kart 7 from USA rev0 I had to install the CIA update, or I could have used the freeshop to update it but not the eshop, you can't install legit updates for a CIA you have to use the CIA update file or use freeshop.
I know this might be confusing to understand so let me make it simple
Rev0 = game without update (you need to update it yourself)
Rev1 = game with update preinstalled so you'll be on v1.1 automatically
Hoped this helped