Key Mapping

Hello Experts,

I have a query at Key mapping level.

Please read my example.

I have 2 systems SYS1 and SYS2.

Sys1 data

country code

-

India India

Sys2 data

country code

-

India IND

While uploading data from sys1

clone field code, map it to remote key

while uploading data from sys2

clone filed code, map it to remote key AND in matching tab select only country and 'update NULL fields only'

Now in DM i can see

country code

-

India India

when i select 'edit key mapping'i can see

2 rows with 'default' cheked for both records.

Well, i have 2 questions based on the above excersie.

1) why should we use ' 'update NULL fields only'' while uploading 'sys2' data???

2) what does 'edit key mapping' table explain us?

Kind Regards

John