From the Authorize.net website
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[/I]
What does "validation is optional" mean?