The .NET SDK uses the NewtonSoft JSON Framework for serialization, and developers can use custom ContractResolvers to modify or ignore fields during serialization. A custom resolver allows developers to strip out metadata fields like "_id" from documents before inserting them into Couchbase. The resolver can be used in conjunction with extension methods to extract the key value for the "_id" field and ensure it is not persisted with the content. This approach enables developers to separate data from metadata, which is useful for optimizing storage and performance. By using a custom ContractResolver or an extension method, developers can achieve this separation easily, making it easier to work with Couchbase and its JSON document format.