In Section 1.5.2 page 13 you mention "each concept in the domain must belong to only one group" speaking about taxonomy and ontology modelling. I do not believe it is true. Though it might be true for a classification (which I believe is different from a Taxonomy).
I agree with you, and I annotated this comment. For some cases in the biomedical domain, I noticed that the exclusivity was a mandatory requirement, but this principle is not applicable in any case.
In Section 1.5.2 page 13 you mention "each concept in the domain must belong to only one group" speaking about taxonomy and ontology modelling. I do not believe it is true. Though it might be true for a classification (which I believe is different from a Taxonomy).