diff --git a/static/skin/block_external.js b/static/skin/block_external.js
index ed6b1ba11..b6e9aa55b 100644
--- a/static/skin/block_external.js
+++ b/static/skin/block_external.js
@@ -1,3 +1,4 @@
+// `block_path` variable used by openzim/warc2zim to detect whether URL blocking is enabled or not
 var block_path = "/catch/external";
 // called only on external links
 function capture_event(e, target) { target.setAttribute("href", encodeURI(block_path + "?source=" + target.href)); }