Portfolios are an excellent way of grouping domains. You can create an unlimited number of portfolios and assign an unlimited number of domains to each. Portfolios are especially helpful within the Domain Manager as you can select domains to update (like renewing, updating domain contacts, setting name servers, etc.) based upon a portfolio selection. For example, if you wanted to renew all of the domains in a specific portfolio, you could select that portfolio, check all of the domains, and then click "Renew".