XSD offers a variety of possibilities to define constraints. However, in some rare exceptions it is not possible to express an important rule in XSD. Rules that do not fit into XSD have to be expressed by words, semantically.
Semantic constraints are even important as XSD constraints. If they are ignored, security issues may arise.
Elements with semantic constraints are listed in Category:Semantic constraints. On the element documentation pages the semantic constraints can be found in a dedicated chapter below the syntactic constraints.
Every application of railML® has to be checked not only on XML conformity but also on obediance of the semantic constraints.