multitenancy domains

I have installed a fresh version of FSPBX. I am familiar with FusionPBX. I am struggling to understand the right way to implement multitenancy. What is the difference between domain and company? I can create a domain the "old" way under Advanced. But I don't see it as an option to select on...

I am confused about domains for the purpose of multi-tenancy. Are they necessary? Can they be namespaces? Is there a way to implement them without registering for a domain name and getting separate IP addresses? What does a home office do when the ISP only assigns one IP address that must be...