Break Glass Vs Request To Exit

When using a electromagnetic lock or other type of hardware that does not allow free egress by mechanical means, it is considered a good practice to provide two methods of unlocking the door. This is commonly a request-to-exit (REX) motion detector, plus some type of manual door release push button or door release pull station.

The requirement to use two independent methods to unlock the door is also often required by building codes, depending on what version of the code is being used and how the local inspector interprets it. 

In addition to Mr. Silva's comment on Request To Exit, 'glass break' sensors are typically an intrusion sensor that trigger when the acoustics of glass breaking are detected.  The idea that the intrusion alarm trips if someone is outside breaking a window or glass lite near a door is the general application, not for access control like a 'Request To Exit'.

Some people call pull stations "break glass" stations because they often have glass rods installed in them that serve as indicators. When you pull the pull station handle, you break the glass rod, providing a positive indication that the handle was pulled. When the pull station is reset, a replacement glass rod is installed.

I believe that the SDC 492 pull station shown above has such a rod. 

And then of course there is one of these very old style pull stations where you actually used a small hammer to break a small glass window to activate the pull station. Note the words "Break Glass" written on the front.