That's the long and short of it- they're 'ac' capacitors, actually they're two polarized capacitors back-to-back, inside a common case.
Polarized electrolytic capacitors are for DC-only applications like DC supply filtration. IF you've used a DC capacitor in an AC application, you'll know because of the hissing noise, smoke emmission, and a violent little explosion.
And another note- motor START capacitors and motor RUN capacitors are internally different. Start caps will carry large current for a short timeperiod, run caps will take moderate currents continuously. IF you put a START capacitor in a RUN application, expect his, smoke, and bang as noted above.
Some OTHER rules:
Capacitors store electric charge using chemical process. Don't EVER assume that a capacitor's terminals are safe to touch, EVEN if you've discharged them previously, as you'll learn that their action is not always complete, that after discharge, a residual charge will build because the electrolysis is still happening. The only thing that will assure a 'safe' capacitor, is having the terminals shorted at all times OTHER than when it's in-circuit, and when in-circuit, there's a 'bleeder resistor' connected to keep it discharged.
Motor START current, particularly through that start cap, is very high. So make sure that whatever you use as your start-current path is really stout.
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