What does the orientation signify? [ardrone_autonomy]

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I am currently looking at the legacy navdata of the AR.Drone's ROS package messages, I just noticed these few things:

# left/right tilt in degrees (rotation about the X axis)
float32 rotX

# forward/backward tilt in degrees (rotation about the Y axis)
float32 rotY

# orientation in degrees (rotation about the Z axis)
float32 rotZ

Now my question lies in the fact that I do not understand what the global coordinates are. What is the x-axis, y-axis and z-axis in terms of the drone? Is it that the axes are set when the drone takes off?

Another question lies in the fact that I do not understand what float32[] tags_orientation means:

#Tags in Vision Detectoion
uint32 tags_count
uint32[] tags_type
uint32[] tags_xc
uint32[] tags_yc
uint32[] tags_width
uint32[] tags_height
float32[] tags_orientation
float32[] tags_distance

How can orientation be an array of floats?

What would be the difference between something like this and the data from let's say the IMU data? I don't want too detailed an answer. Just good pointers to get me started on material that can guide me there.