The CESR genus version this group was framed under. The same code
denotes different groups by genus major (e.g. v1 -C is
NonTransReceiptCouples, v2 -C is the AttachmentGroup wrapper), so a
consumer must know the genus to interpret code.
Typed children; empty for opaque quadlet frames and unknown codes.
A counter-framed attachment group (e.g. controller sigs
-A, quadlets-V).