Can be used with standardized derivatives vs. the MaturityDate (54) field. Month and Year of the maturity (used for standardized futures and options).
YYYYMM (e.g. 199903)
YYYYMMDD (e.g. 20030323)
YYYYMMwN (e.g. 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
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).