An auto-response rule is a set of conditions for sending automatic email responses to lead or case submissions based on the attributes of the submitted record. Applicable leads include those captured through a Web-to-Lead form. Applicable cases include those submitted through a:
Self-Service portal
Customer Portal
Web-to-Case form
Email-to-Case message
On-Demand Email-to-Case message
You can create as many response rules as you like based on any attribute of the incoming lead or case, but only one rule for leads and one for cases can be active at a time.