diff --git a/src/hb-subset.h b/src/hb-subset.h
index cdcc9bd2a..be770e4a3 100644
--- a/src/hb-subset.h
+++ b/src/hb-subset.h
@@ -41,12 +41,32 @@ typedef struct hb_subset_input_t hb_subset_input_t;
 
 typedef enum
 {
+  // If set hinting instructions will be retained in the produced subset.
+  // Otherwise hinting instructions will be dropped.
+  // Defaults to true.
   HB_SUBSET_FLAG_HINTING =		    1,
+  // If set glyph indices will not be modified in the produced subset.
+  // If glyphs are dropped their indices will be retained as an empty
+  // glyph.
+  // Defaults to false.
   HB_SUBSET_FLAG_RETAIN_GIDS =		    2,
+  // If set and subsetting a CFF font the subsetter will attempt to
+  // remove subroutines from the CFF glyphs.
+  // Defaults to false.
   HB_SUBSET_FLAG_DESUBROUTINIZE =	    3,
+  // If set non-unicode name records will be retained in the subset.
+  // Defaults to false.
   HB_SUBSET_FLAG_NAME_LEGACY =		    4,
+  // If set the subsetter will set the OVERLAP_SIMPLE flag on each
+  // simple glyph.
+  // Defaults to false.
   HB_SUBSET_FLAG_SET_OVERLAPS_FLAG =	    5,
+  // If set the subsetter will not drop unrecognized tables and instead
+  // pass them through untouched.
+  // Defaults to false.
   HB_SUBSET_FLAG_PASSTHROUGH_UNRECOGNIZED = 6,
+  // If set the notdef glyph outline will be retained in the final subset.
+  // Defaults to false.
   HB_SUBSET_FLAG_NOTDEF_OUTLINE =	    7,
 } hb_subset_flag_t;