The key refers to your second, or internal rollover line. No longer do you need to be concerned about hanging up one call only to discover that another call you were waiting for is now on Voice Mail. All incoming calls automatically rollover to this second line when your first line is busy. The caller ID of the incoming call on Line 2 can be viewed in the display, affording you the opportunity to pick it up before it goes to Voice Mail.
This key can also be used to place a call within the SIUE telephone system. You can press this key then dial; or you can predial, then press this key to actually send the digits. A third option is to simply lift the handset to automatically select the Primary Directory Number key, or the Internal Rollover Key if you have another call on the Primary Directory Number key.