UDF CD-ROM in Nero Burning ROM 5.5

When you open “New Compilation / Write CD” dialog in Nero Burning ROM, and you select “UDF” tab, you see the “File system options” which include “UDF partition type: Physical / Virtual” and “File system version: 1.02 / 1.50 / 2.01”.

I would like to know what do this options mean. What are differences beetween physical and virtual partition, and 1.02, 1.50 and 2.01 file systems.

Thanks.