How does system recognize changes for delta?

Hai all,

If there are invoices in R/3 and are loded to BW. We load them first initializing delta. Now for recording changes in these invoices, deltas are used.

My question is:

How does the system know that an invoice has been changed?

Is it merely based on that the system see any change(whether its small or big) in an invoice or is it only when there are changes in KEY FIGURES?

I have some invoices in R/3 that are changed and I am loading them to BW. The changes in invoices are date fields and some data fields but not in key figures. When I am trying to load delta, the system says no records changes since the delta initialization... even after 2 days after the invoices change.

Can somebody please help me with this?

Thanks a lot.