From the Authorize.net website
http://www.authorize.net/resources/pcicompliance/


PCI Level 4 Merchants

Less than 20,000 Visa or MasterCard e-commerce transactions per year, and all other merchants processing up to 1 million Visa or MasterCard transactions per year

Recommended annual PCI Self-Assessment Questionnaire and quarterly network scan. Merchant qualified independent scan vendor

Note: While compliance is mandatory for Level 4 Merchants, validation is optional but strongly recommended

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What does "validation is optional" mean?