abstract

This is an abstract class & cannot be instantiated.

What it means

My other question is...
I can create line by using different classes like curve, nxobject,diaplayableobject,line etc

Which one will be preffered ?

"This is an abstract class & cannot be instantiated" means that this class serves as a framework for other classes to inherit from. Sometimes this parent class will define properties and methods that the child classes must implement, but will not itself have a complete implementation of these methods. This class cannot be instantiated because it won't work on its own, a child class must inherit from this class and provide its own implementation.
Check out these links:
http://msdn.microsoft.com/en-us/library/ms973803.aspx
http://support.microsoft.com/kb/307222

How many different ways did you find to create a line?
The CreateLine method has many variants for input, but it will always end up with a line object being created.

So all kf classes are abstract classes

I don't know enough about Knowledge Fusion (KF) to give a definitive answer on that.