Internal contacts are contacts inside your organization or that should be treated as more protected.
In SigParser, emails between two internal contacts are not displayed. Additionally, the subject lines are hidden. This makes SigParser more focused on external contacts.
A couple minutes into this video it covers setting up internal contacts.
You can configure which contacts are seen as internal vs external by configuring a set of email address rules.
Settings -> Internal Contacts
You define pattern for each one you want to match.