A domain name portfolio is the set of domain names you own or the set of domain names you represent as a Seller's Agent or Broker.
When you own the domain names, the portfolio of domains is an investment. You might produce income from your portfolio by leasing the domain, earning ad revenue or affiliate income, or selling the domains.
When you represent the Seller of a domain name, the portfolio of names is similar to an Inventory. You earn commission-based revenue at the time of sale.