Read.replacement vs Write in DMD

Hello,
who knows what is exactly the difference / the added-value of using the load type "read.replacement" instead of "write" when updating existing records ?
Both will do a direct write, so what is different ?

The user guide says : "The READ.REPLACEMENT option will update certain fields of an already Loaded record through direct write of only those fields.".
But we don't know what really happens in background.

Thanks !

Thomas

Doing a Write on existing Records will delete every data in all fields but those already defined in the DMD.
Read.replacement keep existing data without overwriting it with "No values"

Thanks a lot :-)