User-defined topology property types

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21.6.2.2 User-defined temporal property types

Furthermore, authors of application schemas may create their own temporal property types that encapsulate temporal types they have defined in accordance with Clause 14. They shall ensure that these properties follow the rules described in 21.2.6. In addition, the target type shall be a bona fide temporal construct. A temporal property type may be a restriction of an existing temporal property type. A temporal property type may support the choice between an inline or a by-reference semantic or it may restrict the use to either inline prohibit the use of the Xlink attributes or by-reference prohibit the containment of the geometry in the feature.

21.7 Schemas defining coordinate reference systems

21.7.1 Introduction

Many of the concrete XML elements defined in the CRS Schemas may be used without Application Schemas, whenever no content extensions or restrictions are needed. An Application Schema shall be used whenever element contents extension is required, and should be used in most other cases to specify needed restrictions. That is, an Application Schema should be defined to extend andor restrict elements as needed for a specific application, or a set of applications, to:  Add elements to contents of existing elements, for recording additional data about that item needed for that application.  Restrict the multiplicity of current contents elements, to eliminate flexibility not needed and perhaps confusing for that application.  Use a different element name, to be more easily understood in that specific application, primarily for elements that will be instantiated many times.  Specify standard contents and contents patterns for selected elements and attributes, as needed to improve interoperability.  Specify standard XML and other documents to be referenced or otherwise used, as needed to improve interoperability. Application Schemas may thus be used for XML document contents extensions, restrictions, or both. Contents extension is expected to be often used to record additional data needed for applications. Contents restriction is expected to be frequently used to restrict contents, in order to increase interoperability and reduce ambiguity when greater flexibility is not needed for applications. Extensions of existing concrete elements may be defined by extending that concrete element. In many cases, restrictions of existing concrete elements may be done by extending the abstract element from which that concrete element is derived, by adding somewhat different but corresponding extensions. An Application Schema may specify a single top-level element for use in an XML document, with the XML elements and types that it uses. That single top-level XML element may be an object with identity, but this is not required. Such an Application Schema will import and build upon the XML Schema components specified in Clause 12. Application Schemas could define additional concrete elements using or extending other abstract elements, if needed. However, an additional concrete element using or extending an abstract element should not be defined if that concrete element is largely similar to an existing element, and thus probably should extend or use an existing concrete element. In many cases, the existing concrete elements that use an abstract element are believed to be