# XMLmind XML Editor - Online Help

### 8.1. What is a document set?

A document set is simply a set of related XML documents. These documents are related because they all contribute to the content of the same deliverable: PDF file, EPUB file, Web site, etc.

When XMLmind XML Editor knows that some opened documents are members of the same set:

- it will more thoroughly check the links which may exist between these documents,

- it will make it easier creating links between these documents.

Modular documents, for example a DITA map referencing a number of topics or a modular DocBook book including a number of chapters, implicitly specify a document set. When you author such modular documents, it is strongly recommended to first open their document set in XXE.

### About document sets and the DITA key-reference mechanism

If your DITA topics make use of attributes `conkeyref` and `keyref`, then opening it as a document set becomes near mandatory. As a matter of fact, when a DITA map contains key definitions, then the corresponding document set acts not only as a link creation/validation context for its member topics, but it also acts as a key space.

When this is the case, please also take the time to review the options of document sets. See Section 5.2.1, “Document Set options”.