Return
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Title
Content type is sealed or read-only -
Description
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. -
Cause
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".
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Resolution
The warning messages can be safely ignored.