in this context, it means that if you want the spray pressure not to be subject to the limit set in the regulator (which shuts off beyond the calibrated setting) you can do so using the bypass, (which effectively bypasses the regulator itself, and prevents it from intervening if the pressure rises above the pre-set level)

Air Filter/pressure control units - Walther Pilot North America
www.waltherpilotna.com/products/.../air-filter-pressure-con...‎
Filter/compressed air regulator type A. 2 regulated connections, 1 unregulated connection. (for automatic spray gun, pressure tank or pump). Filter/compressed ...

University Links - Oklahoma Water Resources Center
water.okstate.edu/documents/.../ConservationIrrigationTraingCtr.PDF‎
in the middle have a fixed spray, and the rotator is a hybrid of the two. The rotor and the spray head on the right are working at unregulated pressure. 3425 W.

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Note added at 4 days (2014-04-21 16:05:34 GMT)
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to clarify, the pump stops if the pressure drops (not rises) below the level set on the regulator.