Lead assignment rules versus Flow
Key Takeaway
Use a lead assignment rule to specify who owns a lead. Use a flow for the other work that should happen when a lead is created or changed.
Lead assignment rules and Flow both automate work in Salesforce, but they solve different problems. A lead assignment rule has one job: it decides which user or queue owns a lead by checking the lead against an ordered list of criteria. Flow is a general automation tool that can automate many kinds of tasks. A flow can run when a record changes, on a schedule, or when a user clicks something.
How a lead assignment rule decides the owner
A lead assignment rule can assign leads no matter how they arrive: created manually, generated from Web-to-Lead forms, or imported with the Data Import Wizard.
Each rule is made of rule entries. Each entry has an Order number and criteria, and it names the user or queue that gets the lead.
A rule entry can set its criteria in one of two ways. One way is to choose "criteria are met" and add filter criteria, for example that a field equals a value. The other way is to choose "formula evaluates to true" and enter a formula that returns True or False.
Salesforce checks the entries in order. At the first entry whose criteria match, it assigns the lead and stops checking. Entry order therefore decides the owner when a lead matches more than one entry.
Only one lead assignment rule can be in effect at a time, so only the active rule runs. Orgs typically create one rule per overall purpose, such as one rule for imported leads and a different rule for web-generated leads.