The Add on Attachment type (category) can also have several variables to include units that allow you to use your own handpiece photo #1 (Link to page) or units that come with their own motor (Link to page). In either case this type of "add on attachment" will generally hold the motor handpiece (B) stationary so that you can move the model and tilt platform (A) under the rotating RCT, this arrangement will only allow you to move in two Axis, "X" which is side to side and "Y" which is back and forth.
As the cutting advances to the right it is called X+, as it advances towards the left it is called X-
As the cutting advances away from you it is called Y+, as it advances towards you it is called Y-
The stand alone Milling Machine (#2) is specifically designed to perform milling procedures (link to page) i.e., it is not an upgraded surveyor . Generally this type of milling machine will have an articulated (jointed) arm (C)&(D) that allows it to freely move in both "X" any "Y" axis i.e., both side to side and back and forth as the model (A) is locked in place. The additional feature of this milling machine is the ability of the motor to vertically raise and lower (B), varying the vertical height even while milling i.e., what we call "milling on the fly".
The "Z" axis is simply vertically up or vertically down for both milling or
drilling.
In summery all three axis allow the cutting
to occur while moving side to side, back and forth, or up and down.