The continuous duty ratings means that the coil can remain energized indefinitely without damage to the coil. Intermittent duty coils have more power which improves switching characteristics, but because more power is generated than the coil can dissipate, they cannot be energized indefinitely. There are different intermittent duty ratings for the various series:
J46 Series
Prolonged: Up to 90% duty cycle; 50 minutes maximum on time; a little more force than continuous duty
Intermittent: 50% duty cycle; 15 minutes maximum on time; more force than continuous and prolonged duty
High Power: Up to 25% duty cycle; 3 minutes maximum on time; the highest force available.
Plastic Case & Metal Case series
Heavy Duty Intermittent: 25%; 10 minutes maximum on time
Intermittent: 25% duty cycle; 2 minutes maximum on time; more force than HD intermittent