What term specifies a filter's maximum ability to reject signals outside its passband?
Why Ultimate rejection is the maximum attenuation a filter can provide to signals well outside its passband, expressed in dB. Every real filter has a floor: stray coupling around the filter and component limitations mean a strong out-of-band signal eventually leaks through at some level, and that leakage level sets the ultimate rejection figure. A crystal or IF filter might be specified as, say, 80 dB ultimate rejection, meaning nothing outside the passband is attenuated more than that no matter how far removed in frequency.
Watch out Rolloff is the tempting one, but it describes how steeply the response falls off near the passband edges (dB per octave or per kHz), not the ultimate attenuation floor far out; insertion loss is the small attenuation the filter adds inside its passband, and notch depth applies to a single rejected frequency.
Ultimate rejection = the best the filter can ever do far out; rolloff = how fast it gets there.