The momentum acceptance defines the maximum energy excursion that a particle can have before it is lost from the machine. The acceptance is given relative to the reference momentum. A typical optics with small ß* has an acceptance of 1% (LEP 0.7%). In general one has to consider the dynamic aperture which defines the limits for the stable particles in 6-D phase space (transverse and longitudinal).
Due to the large energy spread introduced by beamstrahlung with the very dense FCC-ee beam, the FCC-ee requires an acceptance of 2% at 175 GeV, and at least 1.5% at 120 GeV. This is a challenge for the IR optics and so far there is no optics that satisfies the requirements.