For an HTML document to be valid, the value for all element identifiers need to be unique. When content is transcluded, the identifier is prefixed by the title of the document. This way each element is unique. In case the same element is transcluded multiple times or in some use cases the identifier needs to be defined manually, there may be duplicate element identifiers on one page. 

To prevent duplicate identifiers, this property, when set to true, ensures that each document is rendered in the same identifier context. The property can also be used on page level.

The default value is false since the use cases where duplicate identifiers are a problem is quite small. To ensure uniqueness the content needs to be re-rendered which has a resource impact that may be significant. Users should typically use this feature on page level where duplicate identifiers are actually a problem.