Those numbers refer to the amount of light and dark that the plant receives (18 hours of light / 6 hours of darkness). Anything above 14 hours of light per day will keep the plant in vegetative mode-- growing leaves and not buds. When you want to switch to flowering you reduce the cycle to 12/12.
Cannabis plants grow without hassle under 24 hours of continuous light. There has been no controlled recordings of increased hermaphroditism or problems-- most growers now use the full 24/0 cycle.
However, 18/6 does have it's advantages:
- Reduced electricity costs. When you have a good mother plant collection and clone cycle, you don't need the increased growth the extra light provides. Lowering the cycle will save money.
- Allows the equipment and room to cool down. Those 6 hours of darkness can be timed to happen during the hottest part of the day, providing some relief for growers in hot climates.