Delta update from DSO to InfoCube

Hello,

I need your suggestions and inputs regarding standard delta update from Data Store to Cube

DSO Dates has three fields as key  say - A, B and C  There is one field " E" which is getting updating from different sources (could be sometimes # when not mapped)

Now, cube has E as part of key ( say 10 fields as key but does not include A, B, C )

While updating delta from DSO to Cube , There are multiple lines , one of them including # value for Field E. ( which is not required and is incorrect i believe )

So now how can I avoid this # being updated as key in Cube.

And Also i want to understand that in which scenarios we must not choose DSO to be the source of cube.

Thanks

Regards,

Shipra