Now that I have written all of that out, I think I get how it works. In customer shelf life groups, you can choose a Fefo shelf life group and then set an overriding number of days.
So, you can say, all items that have a minimum shelf life of 21 days, for this customer make shelf life 14 days.
It's not helpful for my client though, as they would like to select individual items to change shelf life.