实际的应用程序可能会使用符号标识符来定位适当的内容处理程序并使用实体的系统标识符作为其输入。

A real-world application might use the notation identifier to locate the appropriate content processor and use the entity's system identifier as its input.

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符号标识符的目的是让XML处理或格式系统更容易找到helper应用程序，该应用程序将处理与符号相关的文件的类型。

Notation identifiers are designed to make it easy for an XML processing or formatting system to locate a helper application that will handle the types of files associated with that notation.

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令牌是单独的关键字、符号和标识符。

Tokens are individual keywords, symbols, and identifiers.

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