An author-defined entity can represent a string of characters or a file name. An entity declaration defines the entity name and the string or file it represents.
Entities are useful for:
Referencing a common string of text. This is useful if there is some likelihood that the text may change or you simply don't want to type it repeatedly. Each place you want the text inserted, you reference the entity name.
Referencing an external file. Entities are required for accessing graphics files. The <figure> and <graphic> elements have a required parameter that specifies an entity name, which refers to a graphic image file.
All entity declarations must be entered before any other markup in your help volume. To include an entity that you have defined, you use an entity reference. Entity references can be used anywhere within your help volume. When you process your help volume with the HelpTag software, each entity reference is replaced with the text or file that the entity represents. " Using Entities" describes how to define and use entities.