The UDI Specifications are organized into multiple books, as described in detail in the "Document Organization" chapter of the UDI Core Specification. The UDI Core Specification contains the definitions for UDI services and facilities required in all UDI environment implementations. The other specification books represent independent, optional services which may be common but are not required in all implementations.
The UDI Specifications contain the only "normative" documentation (that is, the official definition of the interfaces). All the other documents are "informative": they provide useful information and background to help in understanding how to use and implement UDI environments and drivers.
For a high-level list of changes between Version 0.86 and Version 0.90, download changes.pdf (updated April 7, 1999).
For a list of errata describing technical errors discovered in the Version 0.90 specifications, see errata.pdf (updated June 25, 1999).