Understanding whether the participants on a message are part of your team (internal) or not (external) is a large part of determining whether it is treated as private in Charlie.
When considering whether a participant is internal or external, Charlie looks at the domain of the participant's email address (the part after the @ sign). If the domain matches one that is listed in your team's Internal Domains in the Team Settings section of Charlie, the participant is considered to be an internal participant.
Charlie knowing which participants are internal and which are external is important for several reasons:
Messages without at least one internal participant are not ingested into Charlie. This is a security feature to prevent unwanted the risk of a third party directing messages into your Charlie account without your consent (the setting controlling this can be modified to allow for these messages to be ingested should you wish to despite the risk).
Messages that involve only internal participants are always treated as private, and thus only visible to their original participants (internal messages often contain private correspondence that should not be visible to non-recipients).
To learn more about how Charlie handles privacy, have a look here.