diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index 3525da2..6d12e43 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -129,6 +129,9 @@
  *
  * \param[out] attributes  The attribute structure to write to.
  * \param key              The persistent identifier for the key.
+ *                         This can be any value in the range from
+ *                         #PSA_KEY_ID_USER_MIN to #PSA_KEY_ID_USER_MAX
+ *                         inclusive.
  */
 static void psa_set_key_id(psa_key_attributes_t *attributes,
                            mbedtls_svc_key_id_t key);
diff --git a/include/psa/crypto_extra.h b/include/psa/crypto_extra.h
index 6ed1f6c..5f413d6 100644
--- a/include/psa/crypto_extra.h
+++ b/include/psa/crypto_extra.h
@@ -154,6 +154,14 @@
  * specified in \p attributes.
  *
  * \param[in] attributes        The attributes of the existing key.
+ *                              - The lifetime must be a persistent lifetime
+ *                                in a secure element. Volatile lifetimes are
+ *                                not currently supported.
+ *                              - The key identifier must be in the valid
+ *                                range for persistent keys.
+ *                              - The key type and size must be specified and
+ *                                must be consistent with the key material
+ *                                in the secure element.
  *
  * \retval #PSA_SUCCESS
  *         The key was successfully registered.