The <display> element is mandatory and lets you determine how a content type should behave when listed in the administrative pages.

Each piece of content is displayed with a link that you click to open that item for editing. It is up to you to define the text of this link, using a mix of plain text and content from the various fields. A simple example:

<display>
  <link>
    <field ref="filenumber" />
    <text>: </text>
    <field ref="title" />
  </link>
</display>

This snippet will take the filenumber field, append a colon and a space, and then append the value of the title field. <field> elements use the ref attribute to refer to a field in the type, and <text> elements are just plain text that you can use to separate values from fields. This concatenated string will then be used as the link in the content list.

In addition, <field> elements can use the format attribute to perform minimal formatting of the field values. Currently only two formatting options are supported:

Putting it all together you could do something like this:

<display>
  <link>
    <field ref="date" format="date" />
    <text> </text>
    <field ref="filenumber" format="bold" />
    <text>: </text>
    <field ref="title" />
  </link>
</display>

This should all go right after the <sorting> section in your XML:

<?xml version="1.0" encoding="utf-8"?>
<contentType id="document">
  <title lang="en" number="single">Document</title>
  <title lang="en" number="plural">Documents</title>
  <title lang="da" number="single">Dokument</title>
  <title lang="da" number="plural">Dokumenter</title>

  <sorting direction="asc" fieldName="filenumber" type="field" />

  <display>
    <link>
      <field format="bold" ref="filenumber" />
      <text>: </text>
      <field ref="title" />
    </link>
  </display>

  <fields>

     ...

  </fields>

</contentType>