diff --git a/src/types/mark.ts b/src/types/mark.ts
index 4d67f5d..b540643 100644
--- a/src/types/mark.ts
+++ b/src/types/mark.ts
@@ -8,6 +8,8 @@ export interface CurrentUserMark {
   currentValue?: string
 }
 
+// Both PdfFileHash and FileHash currently exist.
+// It enables backward compatibility
 export interface Mark {
   id: number
   npub: string