If the Mandatory Key rule is associated with a field, Duco will consider any records as unmatched (rather than partially matched) should the values in the mandatory field differ.
For example, consider the following two records:
The values in the "Trade Date" field for the two records are different. These are two unrelated trades that happened on different days but where the share price was coincidentally the same. Since six out of seven fields match, Duco would assign a match score of 86%, which is probably above the partial match threshold (Duco's default match threshold is 50%), and list this as a partial match.
As we know that two records with different trade dates definitely represent different trades, we can use the Mandatory Key rule to force the two records to be considered unmatched.