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.

JOINT_AXIS

This is an enumeration type constant that refers to each axis of robot with the standard of joint space coordinate system in the robot controller, and is defined as follows.

Rank	Constant Name	Description
0	JOINT_AXIS_1	No. 1 joint or axis of robot
1	JOINT_AXIS_2	No. 2 joint or axis of robot
2	JOINT_AXIS_3	No. 3 joint or axis of robot
3	JOINT_AXIS_4	No. 4 joint or axis of robot
4	JOINT_AXIS_5	No. 5 joint or axis of robot
5	JOINT_AXIS_6	No. 6 joint or axis of robot

Defined in: DRFC.h

typedef enum {
    JOINT_AXIS_1 = 0,
    JOINT_AXIS_2,
    JOINT_AXIS_3,
    JOINT_AXIS_4,
    JOINT_AXIS_5,
    JOINT_AXIS_6,
} JOINT_AXIS;