It is necessary to configure this possibility in the [[Application Context Management]] (ACM) using the {{UBIKSTUDIO}}.
You can find more information on publishing content on the mobile application in [[HowTo:Make an Object appear on the Mobile Client]].
More precisely, a [[Meta Class]] configured in the ACM can have so-called "possible child classes". If one of those is a document Meta Class, you can create such a document object as a child of an object which has the respective parent Meta Class. A Meta Class is a document, when it has a document classification.