I am not sure if this is the place for such a question, but if I am wrong, please do not get offended.
I am reading their book and wonder why they didn't include wh- words like relative pronouns or relative adverbs in relative clauses as cohesive elements.I believe there are cohesive ties such as those like "reference" between the nouns and these wh-words initializing the modifying clauses. Although they state that sentence structure itself is a cohesive unit, they did analyze cohesive ties within sentence boundaries, which means my assumption that their exclusion of this part is because these are part of sentence structure does not hold.
I may have missed important information which seems obvious to others or I may have misunderstood their work. I believe if my proposal works they may have long done so. I guess there are reasons for them not doing so. However, I didn't find their explicit grounds for exclusion of this part.
Great thoughts welcome!