UPO :
Periodic Orbits : They Are Skeleton For Nonlinear Dynamical Systems, Orbit Is An Iteration Of A Map & When It Is Periodic It's Name Become Periodic Orbit.
When The Space Is Hyperbolic Some Of Them Are Repeller Or Attractor And Their Name Become UPO.
Identifing Them Is Open Problem Yet.
You Should Study Much More.