diff --git a/src/hb-subset.cc b/src/hb-subset.cc
index 369d54ac1..66bb5daba 100644
--- a/src/hb-subset.cc
+++ b/src/hb-subset.cc
@@ -64,6 +64,15 @@
  * that is no longer needed. A subset input describes the desired subset. The input is
  * provided along with a font to the subsetting operation. Output is a new font file
  * containing only the data specified in the input.
+ *
+ * Currently most outline and bitmap tables are supported: glyf, CFF, CFF2, sbix,
+ * COLR, and CBDT/CBLC. This also includes fonts with variable outlines via OpenType
+ * variations. Notably EBDT/EBLC and SVG are not supported. Layout subsetting is supported
+ * only for OpenType Layout tables (GSUB, GPOS, GDEF). Notably subsetting of graphite or AAT tables
+ * is not yet supported.
+ *
+ * Fonts with graphite or AAT tables may still be subsetted but will likely need to use the
+ * retain glyph ids option and configure the subset to pass through the layout tables untouched.
  */
 
 static unsigned