HIL refers to the kind of testing done to study the interactions between the software and the hardware.  In a typical HIL setup, one piece of hardware (e.g., the ECU) is connected to a software model of the rest of the system.  Tests are performed on the HIL to reduce the number of expensive prototypes needed for production level testing.  Emmeskay has helped customers setup their HIL systems on numerous projects for OEM's and Tier 1 suppliers, including work on an off-road vehicle, a fuel-cell controller and a high-voltage battery system.