There are no required CDF rVariables. The first eight characters of rVariable names must, however, be unique for NSSDC applications. Names longer than eight characters will be truncated when displayed.
The optional rVariable EPOCH is used by many NSSDC applications to store date/time values. Section 2.2.10 defines an EPOCH value. The record variance should be VARY, each dimension variance should be NOVARY, the data type should be CDF_EPOCH, and the number of elements should be one (1). If a CDF contains an EPOCH variable, the following variable names should not be used: TIME, YEAR, MONTH, DAY, HOUR, MINUTE, SECOND, MSEC, IYEAR, IMONTH, IDAY, IHOUR, IMINUTE, ISECOND, and IMSEC.