The Signed-up segment works hand in hand with our FS.identify call.
FullStory considers "anonymous" users to be those that don't have a unique user ID assigned via FS.identify, and "signed-up" users to be those who do have a UID.
Note that it is possible to pass user information to FullStory, such as Display Name 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.