Can be used with standardized derivatives vs. the MaturityDate (54) field. Month and Year of the maturity (used for standardized futures and options).
YYYYMM (i.e. 99903)
YYYYMMwN (200303w) for week
A specific date or can be appended to the MaturityMonthYear. For instance, if multiple standard products exist that mature in the same Year and Month, but actually mature at a different time, a value can be appended, such as "w" or "w2" to indicate week as opposed to week 2 expiration. Likewise, the date (0-3) can be appended to indicate a specific expiration (maturity date).