The OPC UA Specification is a multi-part specification and consists of the following parts: 
In contrast to the COM based specifications the UA specifications are not pure application specs. They describe mainly UA internal mechanisms which get handled through the communication stack and are only of interest for those who port the stack to a specific target platform or those who want to implement their own UA stack.
The application developer will code against the OPC UA API and therefore will mainly use the API documentation instead. Nevertheless, part 3, 4, and 5 may be interesting for the application developer as well.