Can be used with standardized derivatives vs. the MaturityDate (541) field. Month and Year of the maturity (used for standardized futures and options).

Format:
YYYYMM (e.g. 199903)
YYYYMMDD (e.g. 20030323)
YYYYMMwN (e.g. 200303w1) for week 1
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).