Reg: Open and Closed Activities in DTR

Hello Guru's,

I am confused with the Open and Closed Activities in DTR, What are the significance of these activities?

And when does an Open Activity changes to Closed Activity?

And why does an Empty Open Activity gets created?

How does the Open Activities effect the Performance of the system?

In NWDS what is the significance of the Public Part and the dcdef xml files?

Please explain and if possible send the links to better understand these? I tried searching for info but with no luck.

Appreciate the Help.

Thanks & Regards,

Pramod