Hello Daniel
Here the way we use them in our setup:
The main use for the different customers is to set them on their respective instance/site.
So if you connect multiple CheckMK Servers via distributed monitoring, you can separate the whole cmk site by assigning it to a customer.
Every host that is monitored on this site will automatically “belong” to the customer.
By then assigning a user to a specific customer, he will be only able to see the hosts on machines/sites that are assigned to his customer.
Hope that helps
Cheers Jens