Both are traffic sources for visits that come from another website. The difference is who owns that website: Portal sites are other sites in your organization, and Referrer sites are sites outside of it. Keeping the two apart means you can see how visitors move between your own sites without mixing that traffic into external acquisition.
Referrer sites
Referrer sites groups visits that come from a site outside your organization. Piano Analytics identifies these visits from the referrer, meaning the URL of the page the visitor was on just before arriving on your site. The browser sends this URL.
For example, if a partner blog links to one of your articles, visits from that link are counted in Referrer sites. This shows how much traffic third-party sites bring you.
Portal sites
Portal sites groups visits that come from another site in your organization. Piano Analytics recognizes these sites because they belong to your organization's scope. Visits from them are counted as traffic between your own sites, not as external referrals.
For example, if your organization publishes a news site and a sports site, visits from the sports site to the news site are counted in Portal sites. This shows how much traffic your own sites send to each other.