Sentence:  It seems equally clear that the ‘Complement + Prepositionʼ order illustrated in (172) (a) is likewise highly marked, and hence subject to heavy restrictions on its use. And sure enough, this does indeed seem to be the case: for one thing, forms such as "thereafter", "herein", "whereby" are stylistically highly marked (e.g. they are only used in particular <WSD>registers</WSD> such as legal language). 
Meaning of the word marked with <WSD> tags:  (language register)  A style of a language used in a particular context.