DSS keys are used to monitor other people's lines. This is the preferred option for Manager/Assistant setups, and is similar to a BLF (Busy Lamp Field). Note that whenever a device has a DSS key, it will also have to have a primary line key labelled My Line.
Suppose your extension 19270 is the primary line on your phone, and you have a DSS key of 88667:
When 19270 is called, the key labelled My Line will flash. Pick up the receiver to take the call
When 88667 is called, the key labelled 88667 will flash. Press the button to pick up the call
If someone else with a DSS key of 88667 has picked up the call, the light will be steady
If the light is not flashing or lit, press 88667 to ring 88667 (it works like a speed dial)