How do recommendations work on the common app?

I’m a little confused about how it works. I know you invite them and they get an email, but the common app says “Please note that this individual will only receive an invitation email once you assign him or her to a college.” Which makes me think every time you assign them to a college, they get an invite, but that seems a little weird.

I’m thinking, you invite them, assign them to a school, they get an email, write a recommendation, and then you can assign the recommendation to other schools later. Is that how it works?

Basically what I mean is - do they get an email and have to submit and rec every time you assign them to a school, or just the first time and then you can assign their recommendations to other schools?