Cross-object matching for leads
Key Takeaway
In orgs created in Summer '17 or later, the Standard Lead Duplicate Rule checks leads against both leads and contacts by default. In orgs created before Summer '17, the admin activates a new duplicate rule in Setup to get cross-object matching.
Cross-object matching lets the Standard Lead Duplicate Rule check leads against two objects, leads and contacts, instead of only against other leads. Whether an org has cross-object matching by default depends on when the org was created.
Matching rules and duplicate rules
Duplicate management has two parts. A matching rule determines how Salesforce identifies records as duplicates. A duplicate rule defines what happens when a user views a record with duplicates or starts to create a duplicate. Each standard duplicate rule includes a corresponding standard matching rule.