Lead assignment rule entries, evaluation order and fallback owner
Key Takeaway
Salesforce checks lead assignment rule entries in order and the first match wins, so put specific entries above broad ones. End the rule with a catch-all entry that has no criteria, because imported leads that match no entry go to the administrator doing the import, not to the "Default Lead Owner".
A lead assignment rule gives each new lead an owner automatically, either a user or a queue, so prospects reach the right person without someone routing them by hand. Salesforce checks the rule's numbered entries in order. The first entry that matches decides the owner. When no entry matches, the lead falls back to a default owner, so a good design plans for that case too.
What a lead assignment rule holds
A lead assignment rule is made of rule entries. Only one lead assignment rule can be active at a time, but that one rule can hold many entries. Organizations typically use one rule for each overall purpose, for example one rule for imported leads and a different rule for web-generated leads.
Lead assignment rules can assign leads that are created manually, that come from Web-to-Lead forms, or that are imported with the Data Import Wizard. When you create a rule, you choose whether it is the active rule for leads created manually and via the web and email.