The Uncategorized category is the default category of our app that is used for all the attachments that have not been placed by users into any other category. For example, once you start with our app, all the attachments are in the Uncategorized category. This category can't be deleted in any scheme. It can only be renamed.
The app behavior is different on Cloud. Due to the architectural limitations we don't have attachment categories. We introduced attachment labels instead. It means that ,after migration, the attachments get the labels corresponding to the category names they had on Server. This is true for all the categories but for the Uncategorized category as this is the default category for attachments that have not been categorized by the user. Thus, the attachments from this default category are migrated to Cloud without labels.