The National Weather Service radar assigns each thunderstorm that develops or moves into the range of a particular site, within the composite reflectivity product, a tracking code. This code consists of one letter followed by one number, such as in X1. Use this code to locate the cell on the radar. The reason some of the characteristics may be different pertains to the vertical level at which the storm was scanned, which depends on the cell's distance from the radar. Best imagery is usually taken close to a radar site, but not above it, allowing for all of the beams sent by the radar to reflect off the cell. Any given thunderstorm may or may not retain the cell identification code between radar scans. This is largely dependent on how consistent the storm is, by means of development and growth, between runs. If the radar believes a new cell has formed, it will identify it with a new tracking code, even though it may have been a detected storm in previous runs that changed structure. Two tracking codes could be assigned to one storm if it appears to the radar that the cell is splitting into two new storms. A linear squall line or bow/comma echo usually has several identification codes. A thunderstorm that is very near or above the radar site may not be detected because the radar's beams are unable to tilt at such an angle. A cell identification code will likely not be assigned to such a cell, which will tend to have poor and inaccurate reflectivity. Even cells that had a tracking code at one point may lose them.