The Signed-up segment is default in your account. This segment is also starred by default in your account.
If you are identifying users with our FS.identify call, then sessions from those users will be in this Segment. For a user's Signed-up status, FullStory considers signed-up users to be those who do have a UID, and anonymous users to be those that don't have a unique user ID assigned via FS.identify.
Note: It is possible to pass user information to FullStory, such as displayName or email address, and still have the user be anonymous. They will not be considered "signed-up" until the user ID has been assigned. This unique User ID is what we use to coalesce sessions for a single user across multiple browsers and devices.