diff --git a/velox/docs/functions/spark/string.rst b/velox/docs/functions/spark/string.rst
index 197bf4e5dd1d..282dacf9c039 100644
--- a/velox/docs/functions/spark/string.rst
+++ b/velox/docs/functions/spark/string.rst
@@ -125,10 +125,10 @@ Unless specified otherwise, all functions return NULL if at least one of the arg
 
     Returns a masked version of the input ``string``.
     ``string``: string value to mask.
-    ``upperChar``: A single character STRING literal used to substitute upper case characters. The default is 'X'. If NULL, upper case characters remain unmasked.
-    ``lowerChar``: A single character STRING literal used to substitute lower case characters. The default is 'x'. If NULL, lower case characters remain unmasked.
-    ``digitChar``: A single character STRING literal used to substitute digits. The default is 'n'. If NULL, digits remain unmasked.
-    ``otherChar``: A single character STRING literal used to substitute any other character. The default is NULL, which leaves these characters unmasked. ::
+    ``upperChar``: A single character STRING used to substitute upper case characters. The default is 'X'. If NULL, upper case characters remain unmasked.
+    ``lowerChar``: A single character STRING used to substitute lower case characters. The default is 'x'. If NULL, lower case characters remain unmasked.
+    ``digitChar``: A single character STRING used to substitute digits. The default is 'n'. If NULL, digits remain unmasked.
+    ``otherChar``: A single character STRING used to substitute any other character. The default is NULL, which leaves these characters unmasked. ::
 
         SELECT mask('abcd-EFGH-8765-4321');  -- "xxxx-XXXX-nnnn-nnnn"
         SELECT mask('abcd-EFGH-8765-4321', 'Q');  -- "xxxx-QQQQ-nnnn-nnnn"
diff --git a/velox/functions/sparksql/Mask.cpp b/velox/functions/sparksql/Mask.cpp
index a3bc1775851d..45ad94307553 100644
--- a/velox/functions/sparksql/Mask.cpp
+++ b/velox/functions/sparksql/Mask.cpp
@@ -23,6 +23,40 @@
 
 namespace facebook::velox::functions::sparksql {
 namespace {
+/**
+ * Masks the characters of the given string value with the provided specific
+ * characters respectively.
+ *
+ * mask(string) -> string
+ * replaces upper-case characters with 'X', lower-case characters with 'x',
+ * and numbers with 'n'.
+ *
+ * mask(string, upperChar) -> string
+ * replaces upper-case characters with provided `upperChar`, lower-case
+ * characters with 'x', and numbers with 'n'.
+ * Upper case characters remain unmasked if provided `upperChar` is NULL.
+ *
+ * mask(string, upperChar, lowerChar) -> string
+ * replaces upper-case characters with provided `upperChar`, lower-case
+ * characters with provided `lowerChar` and numbers with 'n'.
+ * Upper case characters or lower-case characters would remain unmasked if
+ * provided `upperChar` or `lowerChar` is NULL.
+ *
+ * mask(string, upperChar, lowerChar, digitChar) -> string
+ * replaces upper-case characters with provided `upperChar`, lower-case
+ * characters with provided `lowerChar` and numbers with provided `digitChar`.
+ * Upper case characters, lower-case characters and numbers would remain
+ * unmasked if provided `upperChar`, `lowerChar` or `digitChar` is NULL.
+ *
+ * mask(string, upperChar, lowerChar, digitChar, otherChar) -> string
+ * replaces upper-case characters with provided `upperChar`, lower-case
+ * characters with provided `lowerChar` and numbers with provided `digitChar`
+ * and all other characters with provided `otherChar`.
+ * Upper case characters remain unmasekd if provided `upperChar` is NULL.
+ * Lower case characters remain unmasekd if provided `lowerChar` is NULL.
+ * Numbers remain unmasekd if provided `digitChar` is NULL.
+ * All other characters remain unmasked if provided `otherChar` is NULL.
+ */
 class MaskFunction final : public exec::VectorFunction {
  public:
   void apply(