We all know that content types can be configured to be “read-only”. When you use the content type hub and consume one or more content types from that hub, these will also be read-only. When a content type becomes read-only, it cannot be modified (duh…..). Which leads to some interesting scenarios….
First of all. Let’s say we have a document library which has the default “Document” content type and a published “CT hub document”. The first isn’t read-only, but the latter is. When you add a column to this library, it will be added to the Document and views. But it will not be added to the CT hub document. Which isn’t nice, because your users will not be able to edit this column using the “Edit properties” option. The column will be added to the Document content type though.
From a management perspective this makes sense. The content types are managed using the hub and it shouldn’t be possible to edit them on a site collection level. But there are two situations when it is possible to add additional metadata to the read-only content types. And it is out of the box.
1. Add enterprise keywords in the library. When you enable and then add enterprise keywords, all content types will be enabled. Even the read-only ones.
2. Use the Shared Columns option of the Document Set. Yes: the (additional) columns of a document set will be added to all (read-only) content types within the set.
It’s even possible to “add” a Lookup field to a published ‘read-only’ Content Type.
See this blogpost: