Where do assembly instruction "Intel group" categories originate?

During development of my fork of an 86-DOS application, lDebug, I came across categories of instructions referred to as "Intel groups". Specifically, group means something related to a set of instructions using the same 1 or 2 opcode bytes, differentiated by the /r field of the ModR/M byte.

The "Intel group" categories go all the way back to FreeDOS Debug 0.95 released in 1997, and can be seen in that revision of my fddebug repo:

/*
 *  Here are the tables for the main processor groups.
 */

struct {
    int seq;    /* sequence number of the group */
    int info;   /* which group number it is */
}
    grouptab[]  = {
        {0x80, GROUP(1)},   /* Intel group 1 */
        {0x81, GROUP(1)},
        {0x83, GROUP(2)},
        {0xd0, GROUP(3)},   /* Intel group 2 */
        {0xd1, GROUP(3)},
        {0xd2, GROUP(4)},
        {0xd3, GROUP(4)},
        {0xc0, GROUP(5)},   /* Intel group 2a */
        {0xc1, GROUP(5)},
        {0xf6, GROUP(6)},   /* Intel group 3 */
        {0xf7, GROUP(6)},
        {0xff, GROUP(7)},   /* Intel group 5 */
        {SPARSE_BASE + 0x00, GROUP(8)},     /* Intel group 6 */
        {SPARSE_BASE + 0x01, GROUP(9)}};    /* Intel group 7 */

/* #define  NGROUPS 9 (this was done above) */

struct {    /* sparse groups */
    int seq;    /* sequence number of the group */
    int info;   /* which group number it is */
}
    sp_grouptab[]   = {
        {0xfe, SGROUP(1)},      /* Intel group 4 */
        {SPARSE_BASE+0xba, SGROUP(2)},  /* Intel group 8 */
        {SPARSE_BASE+0xc7, SGROUP(3)},  /* Intel group 9 */
        {0x8f, SGROUP(4)},      /* Not an Intel group */
        {0xc6, SGROUP(5)},      /* Not an Intel group */
        {0xc7, SGROUP(5)}};

/* #define  NSGROUPS    5 (this was done above) */

I cannot find anything about the "Intel group" categorisation. An exact web search like "intel group 2a" only turns up a repo of the same application. Broader searches don't turn up anything matching the use in these comments.

At no time have I found any "group" categories in the Intel manuals I have read, either. For example, this 486 manual doesn't seem to list groups in its instruction set reference.

Solution

My 1980 copy of the "8086/8087/8088 MACRO ASSEMBLY LANGUAGE REFERENCE MANUAL for 8086-Based Development Systems" already introduced the 'group' concept albeit still somewhat incomplete.

Opcodes 80h-83h and D0h-D3h could be linked to the named groups "immed" and "Shift" because the instructions that belong to the group have a clear common characteristic.
Not so for the opcodes F6h-F7h and FEh-FFh, so they chose a more universal approach and gave these groups the generic name "Grp " suffixed by a number.

Some 6 years later, the 386 manual took another shot at naming these existing groups and decided to rename the old "Grp 1" as "Unary Grp3", and the old "Grp 2" as "Indirct Grp5". Several new groups were added and once again the heterogeneous nature of these groups made the engineers fall back on generic naming.

8086

opcode	group	000	001	010	011	100	101	110	111
80h-83h	immed	ADD	OR	ADC	SBB	AND	SUB	XOR	CMP
D0h-D3h	Shift	ROL	ROR	RCL	RCR	SHL SAL	SHR	-	SAR
F6h-F7h	Grp 1	TEST	-	NOT	NEG	MUL	IMUL	DIV	IDIV
FEh-FFh	Grp 2	INC	DEC	CALL	CALL	JMP	JMP	PUSH	-

80386

opcode	group	000	001	010	011	100	101	110	111
80h-83h	Immediate Grpl	ADD	OR	ADC	SBB	AND	SUB	XOR	CMP
C0h-C1h, D0h-D3h	Shift Grp2	ROL	ROR	RCL	RCR	SHL	SHR		SAR
F6h-F7h	Unary Grp3	TEST		NOT	NEG	MUL	IMUL	DIV	IDIV
FEh	INC/DEC Grp4	INC	DEC
FFh	Indirct Grp5	INC	DEC	CALL	CALL	JMP	JMP	PUSH
0Fh,00h	Grp6	SLDT	STR	LLDT	LTR	VERR	VERW
0Fh,01h	Grp7	SGDT	SIDT	LGDT	LIDT	SMSW		LMSW
0Fh,BAh	Grp-8					BT	BTS	BTR	BTC