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5.2. Units in comment fields

If the units of the keyword value are specified in the comment of the header keyword, it is recommended that the units string be enclosed in square brackets at the beginning of the comment field, separated from the slash ("/") comment field delimiter by a single space character. This widespread, but optional, convention suggests that square brackets should be used in comment fields only for this purpose. Nonetheless, no software should depend on there being units expressed in this fashion within a keyword comment, nor should any software depend on any string within square brackets in a comment field containing a proper units string. This convention is purely for the human reader, although software could be written which would interpret the string only if present and of proper content. If there is an established convention for the units of a keyword, then only those units may be used. An example, using a non-standard keyword, is
EXPTIME = 1200. / [s] exposure time in seconds