  An electron-positron collider operating at a center-of-mass energy $E_{CM}$
can collect events at all lower energies through initial-state radiation (ISR
or radiative return). We explore the capabilities for radiative return studies
by a proposed high-luminosity collider at $E_{CM}$ = 250 or 90 GeV, to fill in
gaps left by lower-energy colliders such as PEP, PETRA, TRISTAN, and LEP. These
capabilities are compared with those of the lower-energy $e^+e^-$ colliders as
well as hadron colliders such as the Tevatron and the CERN Large Hadron
Collider (LHC). Some examples of accessible questions in dark photon searches
and heavy flavor spectroscopy are given.

High Energy Physics - Phenomenology;High Energy Physics - Experiment