This template causes error messages to show only in certain namespaces. Category sorting is set by a sortkey in the template.
Errors on talk and other pages do not show by default but can be enabled by adding a rule to your CSS:
When creating documentation, the error messages can be transcluded so that the messages are automatically updated. Add
|nocat=true to suppress the error category and the class that hides the error in some namespaces. The message transclusion always adds a hard line break at the end.
Originally these were transcluded, but not all MediaWiki pages support such transclusion.