What mean of "F/S" in microwave component?

Hello, I find a GMI VCO the model is V2555C, it's 10GHz microwave VCO, I want buy it use to test my HP5342A frequency counter, but I was not found any datesheet of this VCO, Its had two SMA port, one is RF OUT and other is VT( I thought 'VT' is Voltage tune? ) and 3 pin : +V, GND, F/S. I don't know what mean of this "F/S", does anyone tell me, is 'F/S' had a regular mean in microwave project?Any advice will be appreciated.

Thank you.

Hui

F/S pin appears to be a control for the "switchable output frequencies"

http://www.ebay.com/itm/10-GHz-X-Ban...-/111095420674

Thanks! I think you are right, the F/S pin should be frequency select, it make sense.