A reserve company is a usable piece of fire apparatus that can be placed into service as a replacement for a first-line rig. This is usually done when a rig breaks down or needs regular maintenance. Reserve apparatus may also be activated during a major emergency. Rochester FD assigns separate numbers to reserve companies, but when they are used as first line apparatus, they are dispatched with the regular number of the company that is using them. RFD reserve numbers begin with the letter R. (R24 is Reserve Engine 24.) Newer RFD reserve companies have 100-series numbers. During major fires, RFD may relocate companies to cover stations closer to the fire scene. The relocated companies retain their own unit numbers. Buffalo FD relocated companies take on the number of the company they are replacing, plus an "X" suffix. For example, if Engine 31 were to relocate to Engine 16's station, it would be dispatched as Engine 16X.