Can be used with standardized derivatives vs. the MaturityDate <541> field. Month and Year of the maturity (used for standardized futures and options).
YYYYMM (i.e. 199903)
YYYYMMwN (200303w1) 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 "w1" or "w2" to indicate week 1 as opposed to week 2 expiration. Likewise, the date (01-31) can be appended to indicate a specific expiration (maturity date).