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