You're reading the documentation for an older, but still supported version (GL013300).
For information on the latest version, please have a look at GL013301.

WRITE_MODBUS_MULTI_DATA

This is a structure information to define a Modbus TCP multi-register write request, including target address, slave information, register start index, and count.

BYTE#	Field Name	Data Type	Value	Remarks
0	_szSymbol	char[MAX_SYMBOL_SIZE]		Symbol name
32	_szIpAddr	char[16]		IPv4 string (null-terminated)
48	_iPort	unsigned short		TCP port
52	_iSlaveID	int		Slave ID
56	_iRegType	unsigned char		Register type
58	_iRegIndex	unsigned short		Start register address
60	_iRegCount	unsigned char		Number of registers

Total size: 64 bytes

Defined in: DRFS.h

typedef struct _WRITE_MODBUS_MULTI_DATA
{
    /* symbol name */
    char                        _szSymbol[MAX_SYMBOL_SIZE];
    /* tcp address */
    char                        _szIpAddr[16];
    /* tcp port */
    unsigned short              _iPort;
    /* Slave ID*/
    int                         _iSlaveID;
    /* i/o type */
    unsigned char               _iRegType;
    /* register start address */
    unsigned short              _iRegIndex;
    /* register count */
    unsigned char               _iRegCount;

} WRITE_MODBUS_MULTI_DATA, *LPWRITE_MODBUS_MULTI_DATA;

typedef WRITE_MODBUS_MULTI_DATA
     WRITE_MODBUS_TCP_MULTI_DATA, *LPWRITE_MODBUS_TCP_MULTI_DATA;