Start capacitors give a large capacitance value necessary for motor starting for a very short period of time (usually seconds long). They are only intermittent duty and will fail catastrophically if energized too long. Run capacitors are used for continuous voltage and current control to a motor's windings and are therefore continuous duty. They are generally of a much lower capacitance value.
In unusual circumstances, a run capacitor could be used as a start capacitor, but the values available are much lower than the values usually available for dedicated start capacitors. The capacitance and voltage ratings would have to match the original start capacitor specification. A start capacitor can never be used as a run capacitor, because it cannot not handle current continuously.
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