They are effectively all the same.
"default" assigns swModel to null (because for non-numeric types such as ModelDoc2, it uses null. For numeric types it sets them to 0). You'd almost never use that in practise though.
Using the second option may prevent the compiler from warning about swModel not being defined before being used. But after compilation it is the same as option 3.
Then which is better to use in that 3 options?
Thanks by the way