A PCI merchant is any businesses that accepts credit cards as a form of payment. A PCI service provider is any company that provides a service to merchants for any aspect for their PCI environment. For both Merchants and Service Providers it is important to understand the difference between attestation of compliance (attestation) and PCI DSS compliance (compliance).
The letter of attestation can be found at the following link: https://www.pcisecuritystandards.org/saq/index.shtml. Attestation is different from compliance... Most banks currently make a distinction between attestation and compliance and request validating documents separately. Attestation is in reference to the following sensitive data whether stored electronically or on paper: Full Magnetic Stripe Data, CAV2/CVC2/CVV2/CID, and PIN/PIN Block. All of that data must not be stored in any format after a credit card transaction has been authorized aka post-authorization. To fill out the attestation form, a company must have adequately identified where any CVV information is located. Data discoveries are a typical project that is associated with this step.
To reach compliance a company needs perform all twelve requirements listed in the latest version of the PCI DSS which can be found here: https://www.pcisecuritystandards.org/security_standards/pci_dss_download_agreement.html. The PCI DSS includes attestation requirements and many other information security practices. To validate compliance a company must submit a Self Assessment Questionnaire (SAQ) or conduct an audit, which results in an Report on Compliance (ROC).
Review this blog if there is still confusion about a bank's letter asking for attestation and compliance with different dates and forms.