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The way Recon works is that if a source attribute has a higher precedence than the target attribute then that attribute would be merged from source to target. By default even null values from source would overwite populated values in the target.
Defer if NULL option is used to stop an attribute that would normally be merged to the target if the source attribute is NULL so preserving data if it is going to be overwritten with a null value.
You find "defer if Null" at precedence rule for every attribute