Line monitor button is the same as BLF, just a different name. You can configure Line Monitor button to monitor another line and blind transfer a call to this line. The following is the detail description about Line Monitor button:
Line Monitor: Monitors the status (ringing, on a call, or on hold) of a single extension. Also known as Busy Lamp Field (BLF). It also functions as a Auto-dial (Speed Dial) button to immediately call a user or to immediately transfer a call to a user, without consultation. In addition, it can be used as an Individual Pickup button to pick up a call that is ringing on the specified extension. If multiple calls are ringing, the first received call is picked up. A device can have only one Line Monitor or Phone Monitor to monitor a user. Tip: This feature is useful for an administrative assistant who monitors an extension of an executive, and for co-workers who back up one another during breaks. Note: This button does not reflect the user's activity on other extensions such as Shared Lines and Shared Extensions.