diff --git a/crypto/x509/x509_vfy.c b/crypto/x509/x509_vfy.c
index 6c31dd5a11a75..7d7fe00257318 100644
--- a/crypto/x509/x509_vfy.c
+++ b/crypto/x509/x509_vfy.c
@@ -2356,12 +2356,15 @@ int X509_STORE_CTX_set_trust(X509_STORE_CTX *ctx, int trust)
 }
 
 /*
- * This function is used to set the X509_STORE_CTX purpose and trust values.
+ * Use this function to set the X509_STORE_CTX purpose and/or trust id values.
+ * The |def_purpose| argument is used if the given purpose value is 0.
+ * The |purpose| is unchanged if also the def_purpose argument is 0.
+ * The |trust| is unchanged if the given trust value is X509_TRUST_DEFAULT.
  * This is intended to be used when another structure has its own trust and
- * purpose values which (if set) will be inherited by the ctx. If they aren't
- * set then we will usually have a default purpose in mind which should then
- * be used to set the trust value. An example of this is SSL use: an SSL
- * structure will have its own purpose and trust settings which the
+ * purpose values, which (if set) will be inherited by the |ctx|. If they aren't
+ * set then we will usually have a default purpose in mind, which should then
+ * be used to set the trust id. An example of this is SSL use: an SSL
+ * structure will have its own purpose and trust settings, which the
  * application can set: if they aren't set then we use the default of SSL
  * client/server.
  */