diff --git a/src/xattrs.c b/src/xattrs.c
index df65460..2c10836 100644
--- a/src/xattrs.c
+++ b/src/xattrs.c
@@ -636,6 +636,10 @@ xattrs_matches_mask (const char *kw, struct xattrs_mask_map *mm)
   return false;
 }
 
+/* during extraction xattrs are checked against the user supplied
+   include/exclude mask but if no mask is given then only the user.*
+   domain is included as other domains may be incompatible across
+   platforms. */
 #define USER_DOT_PFX "user."
 
 static bool