The A, B, or C value a signing provider places in a SHAKEN token to state how much it knows about the caller and their right to use the calling number.
Attestation is the signing provider's statement of confidence, carried in the "attest" claim of the SHAKEN PASSporT defined in RFC 8588. It does not say whether a call is wanted or lawful; it says what the originating provider can vouch for about the source of the call and the number it presents.
A, or full attestation, means the provider has authenticated the customer originating the call and knows that customer is authorized to use the calling number. B, or partial attestation, means the provider knows the customer but cannot confirm the customer's right to that specific number, which is common with PBX customers sending numbers from other carriers. C, or gateway attestation, means the provider is passing along a call it received from a source it cannot authenticate, such as an international gateway.
Downstream analytics engines weigh attestation alongside many other signals. An A-level signature from a provider with a poor reputation can still be blocked, and a well-behaved B-level stream can complete normally. What matters is that the level is accurate and backed by a documented process for deciding when a customer qualifies for each value.
Why it matters to carriers
Attestation decisions are where a provider's Know Your Customer process meets the network. Overstating attestation to improve answer rates can expose you to traceback findings and enforcement, while understating it for legitimate enterprise customers can hurt their completion and branded display. A clear, auditable policy for assigning A, B, and C protects both your customers and your certification.