FC should stand for "fluorescent circline", which means the bulb is circular, but it wouldn't surprise me if some manufacturers didn't adhere to that.
In , the bulb code is FC12T5, meaning the tube itself is 5/8" in diameter, but it forms a torus with a 12" outer diameter. In , however, the bulb code begins with FC30, and the 30 indicates that it's a 30W bulb. There doesn't seem to be a lot of consistency on what the number after the FC represents on circline bulbs, sometimes its outer diameter and sometimes its energy consumption.
Great thanks for the info, so basically a FC bulb is one that circular, but there is no standard as to what the number after the FC designation means. Great info thanks for all the help.