Add blurb about why PRF test vectors have two sections

index.bs

@@ -55,7 +55,9 @@ Text Macro: RPS Relying Parties
 Text Macro: INFORMATIVE <em>This section is not normative.</em>
 Text Macro: TRUE <code>true</code>
 Text Macro: WAA WebAuthn Authenticator
+Text Macro: WAAS WebAuthn Authenticator
 Text Macro: WAC WebAuthn Client
+Text Macro: WACS WebAuthn Clients
 Text Macro: WRP WebAuthn Relying Party
 Text Macro: WRPS WebAuthn Relying Parties
 Text Macro: CREATE-METHOD-ARGS (origin, options, sameOriginWithAncestors)
@@ -9551,6 +9553,12 @@ This section lists example values for [=WebAuthn extensions=].
 
 This section lists example values for the pseudo-random function ([=prf=]) extension.
 
+Because the [=prf=] extension integrates with the CTAP2 `hmac-secret` extension [[FIDO-CTAP]],
+the examples are divided into two sections:
+example inputs and outputs for the WebAuthn [=prf=] extension, relevant to [=[WACS]=] and [=[WRPS]=];
+and example mappings between the WebAuthn [=prf=] extension and the CTAP2 `hmac-secret` extension,
+relevant to [=[WACS]=] and [=[WAAS]=].
+
 
 #### Web Authentication API #### {#test-vectors-extensions-prf-webauthn}