What are the characteristics of the WinCC OA API?

Most scada packages will have an API ( Application Programmers Interface ) that exposes some functions like : read tag, write tag. Such an API is implemented especially for customers who want to extend the functionality of the package.
The WinCC OA API is quite different. It is not a special API implemented for customers but it is the one-and-only set of development libraries used to implement all WinCC OA managers. 
There is only one API that is used by:

• ETM to implement WinCC OA and its managers
• Customers to implement specific drivers and/or managers

This implies that developers can produce C++ drivers and managers that are similar to the standard drivers and managers in WinCC OA.