During a site or site collection migration, the following warning messages may be seen in the log:
The content type "Unknown Document Type" at "/sites/siteName" is read only.
The content type "System" at "/sites/siteName" is sealed.
These are Base Content Types which exist on every site collection in SharePoint. These content types are not meant to be altered and are therefore either "Sealed" or "Read only".
The warning messages can be safely ignored.
