The Sensor Enclosure is attached to the outside of the woodworking tool’s power cord and senses when the power tool is turned on.
The Sensor Enclosure connects to the Blast Gate Enclosure.
You need one Sensor Enclosure for each tool you want to auto sense when it is turned on. This would be the case for most all power tools with the exception of maybe a mitersaw where the saw is turned on and off multiple times in milling session.  A mitersaw would use a Manual Switch to activate insted of a Sensor Enclosure.