Each shape in a drawing has a regroup ID (separate from the shape ID), so that the regroup command can find shapes that were once grouped. In order to handle nested cases (e.g. ungroup, ungroup, ungroup, regroup, regroup, regroup), there is a table logging changes to regroup IDs. Each entry has an old ID and a new ID and records the change of all instances of the old ID to the new ID.
The instance of an msofbtRegroupItems record contains the number of entries, and the record itself is just that many FRITs (File Regroup Items).
typedef struct _FRIT // File Regroup item
{ FRID fridNew; FRID fridOld; } FRIT;