FISA allows three vectors of surveillance from the target. What qualifies as a "vector" is a bit vague, but effectively if I am the target of a FISA 1, then everyone I am friends with on FB, any online handles I have comms with, etc count as vector one. Vector two is anyone they come into contact with. Vector three is anyone that set of people come into contact with.
So… In theory, if Carter Page is target, then POTUS is vector one. Anyone in contact with POTUS is vector two. And then there is still a vector three to have fun with.