ISO/IEC 29341-20-4:2017定义了AV数据结构模板（AVDT）XML文档的布局。AVDT文件描述了UPnP AV规范中使用的各种数据结构的格式要求和限制。尽管这些数据结构在适当的服务规范中定义得非常精确，但在大多数情况下，每个数据结构定义都允许一定程度的变化，以适应各个设备之间的差异。 AVDT文档的目的是使每个设备能够（在运行时）描述这些AV数据结构的特定变化。 AVDT文件允许AV数据结构（例如UPnP控制点）的用户减少符合服务规范但与设备特定功能不兼容的数据结构实例的数量。AVDT文档的最终目标是减少因控制点创建超出设备静态（已知）能力的数据结构实例而导致的错误情况。不幸的是，AVDT机制永远不会消除所有可预防的错误情况，但它将通过向客户端提供有关设备特定功能的更多信息来帮助减少这些情况。 如上所述，AVDT文档是由UPnP AV规范之一定义的AV数据结构的机器可读的、特定于实现的变体。对于给定设备，该数据结构的每个实例都应符合该数据结构的规范定义和设备的AVDT定义。 具有讽刺意味的是，AVDT文档既是规范定义的一个更严格的变体，也是一个更宽容的变体。AVDT文档的限制性更大，因为它们限制了规范定义所允许的数据结构的某些方面（例如，每个字段的允许值）。 然而，由于AVDT构造的限制，无法表达规范定义的一些更复杂的需求（例如，数据结构字段之间的微妙相互依赖关系）。因此，符合给定AVDT描述的数据结构实例可能不完全符合规范中定义的所有要求。 AVDT文档可以描述的数据结构类型表示一组（非层次）命名属性值。给定数据结构的一组允许属性名及其允许值由UPnP AV规范之一定义。 这些数据结构的各个实例通过一个XML文档来表示，该文档的元素和属性对应于一组命名属性。换句话说，在对应于特定数据结构的给定实例的XML文档中，每个XML元素和属性都包含特定命名属性的值。 AVDT文档在概念上类似于XML模式，因为这两个实体都标识出现在任何给定文档实例中的XML元素和属性。此外，AVDT文档和XML模式都标识了与特定属性对应的每个元素和/或属性所允许的值。 然而，与XML模式不同，AVDT文档还可以识别两个或多个属性之间的某些依赖关系。例如，一个属性的允许值集可能取决于另一个属性的实际值。这种相互关系很难用XML模式表示。因此，需要AVDTdocument结构。 在各种AV架构场景中，有时需要交换设备功能以确保高级别的互操作性。为了表达参数化功能，AV规范为每个目的定义了各种模板。 设备使用模板并用值填充模板，以反映其在运行时的功能。 AV数据结构模板（AVDT）是定义各种模板的常用结构，称为“数据结构”。这是用XML编写的，每个数据结构都使用AVDT的一个子集来满足必要的需求。

ISO/IEC 29341-20-4:2017 defines the layout of the AV Datastructure Template (AVDT) XML document. An AVDT document describes the format requirements and restrictions of various data structures used within the UPnP AV specifications. Although these data structures are defined very precisely in the appropriate service specification, in most cases, each data structure definition allows for a certain degree of variation in order to accommodate differences between individual devices. The purpose of an AVDT document is to enable each device to describe (at run-time) its particular variation of these AV data structures. AVDT documents allow users of AV data structures (e.g. UPnP control points) to reduce the number of instances of those data structures that comply with the service specification but are not compatible with the device's particular capabilities. The ultimate goal of an AVDT document is to reduce those error conditions that are caused by control points creating instances of a data structure that exceed the static (known) capabilities of the device. Unfortunately, the AVDT mechanism will never eliminate all preventable error conditions, but it will help to reduce them by giving the client more information about the device's particular capabilities. As described above, an AVDT document is a machine readable, implementation-specific variant of an AV data structure defined by one of the UPnP AV specifications. For a given device, each instance of that data structure shall conform to both the specification definition and the device's AVDT definition of that data structure. Ironically, an AVDT document is both a more-restrictive and more-permissive variant of the specification definition. AVDT documents are more restrictive because they limit certain aspects of the data structure (e.g. such as the allowed values for each field) that are otherwise permitted by the specification definition. However, due to limitations of the AVDT constructs, it is simply not possible to express some of the more intricate requirements defined by the specification (e.g. subtle interdependencies between data structure fields). Consequently, instances of a data structure that comply with a given AVDT description may not fully comply with all of the requirements defined in the specification. The types of data structures that can be described by an AVDT document represent a (non-hierarchitical) set of named property values. The set of allowed property names and their allowed values for a given data structure are defined by one of the UPnP AV specifications. Individual instances of these data structures are manifested via an XML document whose elements and attributes correspond to the set of named properties. In other words, within the XML document that corresponds to a given instance of a certain data structure, each XML element and attribute contains the value of a specific named property. An AVDT document is conceptually similar to an XML schema in that both entities identify the XML elements and attributes that appear in any given document instance. Additionally, both AVDT documents and XML schemas identify the allowed values that are permitted for each element and/or attribute which corresponds to a specific property. However, unlike an XML schema, an AVDT document can also identify certain dependencies between two or more properties. For example, the set of allowed values of one property may depend on the actual value of another property. This type of interrelationship is difficult to represent using an XML schema. Hence, the AVDTdocument structure is needed. In the various AV Architecture scenarios, sometimes there is a need to exchange device capabilities to ensure high level interoperability. In order to express the parameterized capability, an AV specification defines various templates for each purpose. A device uses the template and populates it with values to reflect its capabilities at run-time. The AV Datastructure Template (AVDT) is a common structure to define various templates, which are called "Datastructure". This is written in XML and each data structure uses a subset of the AVDT to meet the necessary requirement.