Duplicate rule comparison and user record access
Key Takeaway
If the user who updates a record can't access a field referenced in the matching rule, the duplicate rule doesn't work as expected.
A duplicate rule decides what happens when a user saves a record that looks like one already in Salesforce. The matching rule inside each duplicate rule decides how two records are compared. What a rule finds, and what a user sees, depends on the rule's setup and on the user's access to records and fields.
How a matching rule compares records
The matching equation in a matching rule sets how the fields in a pair of records are compared. Criteria on those fields set how broadly a match is defined.
A matching rule can compare records on the same object, such as lead to lead. It can also compare records on one other object, such as leads that duplicate contacts.