DSO Overwrite and Summation

Hi,

1) When to use overwrite and Summation for key figures in DSO.

2) change log table is used for updating deltas into info cube. My doubt is let say that

in the first load, i have loaded  into cube from DSO

Custid   Revenue

C001      300

DSO has Overwrite function..In next delta load..i have loaded

C001  400.

Since DSO has overwrite function..data in the DSO will be  C001 400 . What data will be loaded into the cube

C001 100  or something else. if C001 100 is got loaded..where we can see this data.

I hope it is clear

Thanks

nikhil