Also in section 1.8 of the 276 spec, it states "Trading partner agreements are used to establish and document the relationship between trading partners. A trading partner agreement must not override the specifications in this implementation guide if a transmission is reported in GS08 to be a product of this implementation guide."
In version 005010, the value of “U” is the appropriate option when the sender cannot explicitly select either “M” or “F”. While the description of value “U” is Unknown, in this case, Unknown only means that neither “M” nor “F” can be sent. The reason that neither “M” nor “F” can be sent is what is unknown. “U” does not necessarily mean that the gender is unknown to the sender.
X12 will be addressing the need for additional code values for the DMG03 in future versions.