Had a look at that and I seemed to get the hang of it, however, there are some strange things that I don't understand.
1. I have 4 ImageTargets under my MultiTarget, as you described how to do earlier. All these targets have the exact same values, but still one random target(in this case "Drone_Warehouse") has the Z-rotation of 360 degrees in Unity while the others don't, is this just a bug? Here's the part from the XML file.
<Part name="Oxygen_Warehouse" translation="1 0 0" rotation="AD: 1 0 0 90"/>
<Part name="Fissure_Warehouse" translation="2 0 0" rotation="AD: 1 0 0 90"/>
<Part name="Drone_Warehouse" translation="3 0 0" rotation="AD: 1 0 0 90"/>
<Part name="Astronaut_Warehouse" translation="4 0 0" rotation="AD: 1 0 0 90"/>
I could find no bug on this issue, but I did find a comment on an unrelated bug that basically states that visualizations of hand-edited MultiTargets in Unity, Editor or PlayMode, may not match expected results. In other words, if your MultiTarget is not a cube or cuboid, how it behaves in Unity is untested.
Thus, are your observations from the Editor, PlayMode or how it behaves on device? If it is either of the former then the behavior is basically undefined.
2. I don't seem to understand the syntax of the rotation="AD:..... How do I for example change rotation of the Drone target to 90X, 0Y, 180Z in Unity from here? All the above gives me -90X, 0Y, 0Z, except for the drone.
After the "AD:", the first three values are boolean and indicate the rotation axis. Thus, in your example above, each side will be rotated +90 degrees on the X-axis. Although it is not directly documented, you can include multiple sets of these values to rotate on multiple axis. Here is an example:
<Part name="FancyCube.Front" translation="0.002 0.01 0.055" rotation="AD: 0 0 1 -50; 1 0 0 42"/>
Now for the bad news: there is a bug in the SDK when using multiple rotations. One customer reported that "..when start the application the tracking works when the camera see the other targets [with single rotation value set], but not when is looking at the multiple-rotated marker..." The bug has been deprioritized as we only get one user a year who is trying to use MutliTargets like this.
Please let me know how it goes.
Vuforia Engine Support