Complex images contain substantial information, more than can be conveyed in a short phrase or sentence of typical short alt-text. These are typically:
In these cases, a two-part text alternative is required. The first part is the short description to identify the image and, where appropriate, to indicate the location of the long description. The second part is the long description – a textual representation of the essential information conveyed by the image. In the WCAG guidelines for how to manage complex images, reference is made to what's called the "longdesc" html attribute. The status of the attribute has been in flux since the the specifications of HTML 5 were finalized and published in 2008 because it was deprecated and not carried over from HTML 4.0 standards. There was a recommendation in 2014 to include it back in to HTML 5, but currently no modern web browsers support it.
For the equivalent long descriptions for alt-text with complex images, Drupal Web Express is engineered to use main page content, short alt-text settings available in the interface, as well as the caption attribute in the WYSIWYG editor.