Localization Codes are typically (though not necessarily) used with format statements in LabVIEW. You will often see them in National Instruments instrument drivers. Localization Codes are referred to in Table 6-2 of the LabVIEW Function and VI Reference Manual
Typically, you will see a format string such as "%f" or "%d" etc. Sometimes your format string can be something like "%.;%f", where %.; (period followed by a semi-colon) is the Localization Code. This is most often needed on operating systems that do not use English, or at least operating systems that have a decimal symbol other than "." (e.g., German operating systems may have commas as the decimal separator). This can be set in Start » Settings » Control Panel » Regional Settings » Number » Decimal Symbol
on your operating system.
In LabVIEW there is a setting under Tools » Options » Front Panel
called "Use Localized Decimal Point." If this setting is FALSE, the conversion functions (such as string conversion functions) always use a period. If the preference is TRUE, the functions use the system's setting for localized decimal point (that are set as explained above).
Having a default decimal separator other than a period will generate problems, however, if you use the string conversion functions to generate strings for GPIB, message-based VXI, or other communications (DDE, OLE, etc.), because these devices expect decimal delimiters to be periods. You can force the string to use a period as a separator by adding the Localization Code " .;" ahead of the actual format string. This will cause a number such as 3,55 to show up as 3.55.
Alternatively, you can use the Localization Code " ,; " (comma followed by a semi-colon) to force a number such as 3.55 to be converted as 3,55. That is why, when you have a decimal number as a parameter to an instrument, the format string has a ".;" ahead of it. It is good programming practice, since the user can now use this VI on operating systems which do not have the period as the default decimal separator.