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.

BYTE_SIZE

This is an enumerated constant about the size of data to be transmitted during serial communication, and is defined as follows.

Rank	Constant Name	Description
0	BYTE_SIZE_FIVEBITES	5 bits
1	BYTE_SIZE_SIXBITS	6 bits
2	BYTE_SIZE_SEVENBITS	7 bits
3	BYTE_SIZE_EIGHTBITS	8 bits

Defined in: DRFC.h

typedef enum {
    BYTE_SIZE_FIVEBITES = 5,
    BYTE_SIZE_SIXBITS,
    BYTE_SIZE_SEVENBITS,
    BYTE_SIZE_EIGHTBITS
} BYTE_SIZE;