ANYTHING in a text file following a semicolon ; is ignored by the game engine. The line
;Visual
is a comment to help programmers/modders locate the section of Sensors.cfg which pertains to visual sensors.
The values after the semicolon (1.4;0.5) are also comments. They may refer to the value of the parameter in the stock Sensors.dat file. In Airstrike.cfg, they give acceptable ranges for the parameter. In other files, they may refer to something else entirely. They are not used by the game engine, which always stops reading a line in a text file when it encounters a semicolon.
In some cases, the values in the comment might refer to acceptable alternative values. But they serve whatever purpose the programmer/modder who wrote or altered the file intended. So you must infer the intent from context, and should not assume that the values have any particular significance.
Commented lines abound in commands_xx.cfg, where they are just a way to disable a line without removing it. So you can see the history of changes in the commented lines - if you can figure out what was changed when.