diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index a2d547f..5096748 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -130,6 +130,9 @@
  *
  * \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);