The 271 2120C/D loops support codes for Primary Payer, Secondary Payer and Group. Since this loop is used for COB information (among other things), is this instance of Group used to identify the Group of the other insurance, or the medical practice (group) related to the current Information Source coverage? If it is the Group of the other insurance, we assume that the Group Name would be provided in NM103. Then, what code value would be used in NM108 to identify that the Group Number is in NM109? None of the existing values seem appropriate, although "PI" (Payor Identification) seems closest.
The 271 2120C/D NM101 code GW (Group) is used only to identify a medical group (e.g. clinic, IPA); it is not used to identify a group associated with other insurance in a COB situation.
The 2120 loop is used when needed to identify an entity associated with the eligibility or benefits being identified in the 2110 loop.
Group numbers for plans other than the information source in 2100A would be sent in the 2110C/D REF02 with REF01 = 6P (Group Number) and the Group Name can be sent in REF03.