Here is something I found at the Zetron.com site:
In-plant/Out-plant Systems Benefit From Wireless Watchdog
Modern industries like petrochemicals, energy generation, and other industrial applications have hundreds of processes that require equipment to be monitored and turned on or off at key times. In many of these applications, a simple metering device and automatic switch may be all that is required, but often the distances involved call for a remote way of turning devices on and off. For these applications, Zetron created the Model 18/18+ Control Link, a kind of “wireless conduit” that uses radio transceivers instead of wires and provides a versatile method of relaying alarm conditions, status changes, and remote control commands.
This passage implies that 'in-plant/out-plant' is the equivalent of on-site/off-site. The word 'plant' is often used to refer to the physical site of manufacturing/ production.
I hope this is helpful,