The 1st axis defines the length of the machine's base in the direction that the bridge moves (for bridge machines), or the length of the base in the direction that the vertical carriage moves (for horizontal arm machines). This is usually the X axis for most horizontal arm machines, and either the X or the Y axis for most vertical arm machines.
The 2nd axis defines the width of the base in the direction that the carriage moves (for a bridge machine) or the direction the arm moves (for a horizontal arm machine). This is the vertical Z axis for most horizontal arm machines and either the X or the Y axis for most vertical arm machines. This differs from the machine volume. The volume defines the distance the arm can reach. Most arm machines have a volume just over 50% of the width.
The 3rd axis defines the height of the machine from the top of the base to the bottom side of the bridge for a bridge machine, or the extent of the vertical axis for a horizontal arm machine. This is the vertical Z axis for most bridge machines and the Y axis for most horizontal arm machines. At this time only the following axis configurations are allowed:
Generic Bridge with Y, X, Z or X, Y, Z
Generic Arm with X, Y, Z