diff --git a/icu4c/source/common/ucnv_cnv.h b/icu4c/source/common/ucnv_cnv.h
index 6b18bde0be3..5b948f4fbba 100644
--- a/icu4c/source/common/ucnv_cnv.h
+++ b/icu4c/source/common/ucnv_cnv.h
@@ -84,7 +84,12 @@ typedef void (*UConverterReset) (UConverter *cnv, UConverterResetChoice choice);
  */
 typedef void (*UConverterToUnicode) (UConverterToUnicodeArgs *, UErrorCode *);
 
-/* same rules as for UConverterToUnicode */
+/*
+ * Same rules as for UConverterToUnicode.
+ * A lead surrogate is kept in fromUChar32 across buffers, and if an error
+ * occurs, then the offending input code point must be put into fromUChar32
+ * as well.
+ */
 typedef void (*UConverterFromUnicode) (UConverterFromUnicodeArgs *, UErrorCode *);
 
 /*
diff --git a/icu4c/source/common/unicode/utypes.h b/icu4c/source/common/unicode/utypes.h
index 27aad128183..d2c42e9c084 100644
--- a/icu4c/source/common/unicode/utypes.h
+++ b/icu4c/source/common/unicode/utypes.h
@@ -528,9 +528,9 @@ typedef enum UErrorCode {
     U_MEMORY_ALLOCATION_ERROR =  7,     /**< Memory allocation error */
     U_INDEX_OUTOFBOUNDS_ERROR =  8,     /**< Trying to access the index that is out of bounds */
     U_PARSE_ERROR             =  9,     /**< Equivalent to Java ParseException */
-    U_INVALID_CHAR_FOUND      = 10,     /**< In the Character conversion routines: Invalid character or sequence was encountered. In other APIs: Invalid character or code point name. */
-    U_TRUNCATED_CHAR_FOUND    = 11,     /**< In the Character conversion routines: More bytes are required to complete the conversion successfully */
-    U_ILLEGAL_CHAR_FOUND      = 12,     /**< In codeset conversion: a sequence that does NOT belong in the codepage has been encountered */
+    U_INVALID_CHAR_FOUND      = 10,     /**< Character conversion: Unmappable input sequence. In other APIs: Invalid character. */
+    U_TRUNCATED_CHAR_FOUND    = 11,     /**< Character conversion: Incomplete input sequence. */
+    U_ILLEGAL_CHAR_FOUND      = 12,     /**< Character conversion: Illegal input sequence/combination of input units.. */
     U_INVALID_TABLE_FORMAT    = 13,     /**< Conversion table file found, but corrupted */
     U_INVALID_TABLE_FILE      = 14,     /**< Conversion table file not found */
     U_BUFFER_OVERFLOW_ERROR   = 15,     /**< A result would not fit in the supplied buffer */