Based on prior conversations, the following is my understanding of libnetconf's support for RFC 6991 (Common YANG Data Types).
The two YANG modules (ietf-yang-types and ietf-inet-types) are advertised as capabilities.
The extent of direct support by libnetconf, however, is as a part of validation (via the RelaxNG and Schematron libraries.) Validation, when applied, will validate incoming XML data against the appropriate types.
Within libnetconf itself, however, there is no support for types or typedefs. All data transfer is via XML and has no further semantic knowledge. It is the responsibility of the server that uses libnetconf and the shared libraries implementing its models to correctly translate data between the XML representation and a (probably binary) internal representations.