3.4.2 Reserved Keywords for ASCII Table Extension

  These keywords may be used, in addition to the required keywords described in section 3.4.1 , to describe the structure of ASCII table data. They are not required, but if they are used in an ASCII table header, they must be used as described here. These keywords should not be used in other extensions unless they have a function analogous to the one they have in the ASCII table extension. They may appear anywhere between the TFIELDS and END keywords, in any order.

• TTYPEn (character) is the heading for field n. The recommended characters are letters, digits, and underscore. Upper case values are preferred, but lowercase letters may be used, and string comparisons involving the values of TTYPEn keywords should not be case sensitive. The hyphen is permitted but not recommended. While more than one field may have the same heading (value of TTYPEn), the practice is not recommended.
• TUNITn (character) is the physical units of field n. The rules governing the units to be used in ASCII tables are the same as those for a primary data array, as given in section 3.1.1.4.
• TSCALn (floating) is the scale factor for field n, as applied in equation 3.11. The default value is 1.0.
• TZEROn (floating) is the offset for field n (see equation 3.11). The default value is 0.0.
  

  Physical_value = (Stored_value) × TSCALn + TZEROn.   (3.11)

  Note: TSCALn and TZEROn may not be used for A-format fields.
• TNULLn (character) is the representation of an undefined value. The string chosen does not have to be readable in the format specified by TFORMn; for example, a null value of '***' might be used for an I3 field. If the null string given as the value of the keyword does not fill the field, it is assumed to begin at the start of the field, and the part of the field following the string is assumed to be filled with blanks. A table will be more efficiently read if TNULLn is specified only when null values actually exist in the field, thus avoiding unnecessary string comparisons for the other fields.