N. Fix determined by and Fix Level is the method used to determine where the center of the storm was located. There can be more than one method and level shown here at times.
The methods are 1-Penetration, 2-Radar, 3-Wind, 4-Pressure, 5-Temperature.
After the solidus ("/"), you'll find one or two numbers which show at what level(s) the center was found, as follows: 0-surface, 1-1500 ft, 8-850 mb, 7-700 mb, 5-500 mb, 4-400 mb, 3-300 mb, 2-200 mb, 9-925 mb.
In this case it was 12345/7 meaning the center was found by all 5 of the methods to be at the 700mb level.