The Image Recognition part and the Augmented Reality part have different requirements.
The Image Recognition component works with any object, regardless of its real-world geometry, by design. The technology is engineered to recognize not only planar objects such as a book covers, but also rounded objects such as cans and bottles. You can read more about what kind of objects we recognize.
For Augmented Reality experiences, the tracking of the real world object assumes that the object is flat (such as a postcard or a billboard). We suggest not to augment objects that are not planar.