What is a Fire alarm circuit style y?

Style Y is a designation of circuit performance under different states of the circuit. For example: an initiating circuit and a NAC circuit may both be class B, with a panel on one end of the circuit and a resistor on the other. If there is an open on either circuit (the panel doesn't see the EOL), then a trouble is initiated at the panel. However, if there is a short between the two wires on a circuit, the panel will react very differently on the initiating circuit than it will on the NAC. Style Y means that the fire alarm panel will display a trouble on either an open or a short on the NAC circuit, where style 4 means that the Initiating circuit will generate an alarm on the panel in the event of a short, and a trouble in the event of an open.