The syntax is =SUMTOTAL(function_number, reference_1, [reference_2]....)
The function_number arguments informs Excel what you require of SUBTOTAL as
shown in the table below taken from Help.
The value 9 means you want to SUM (rather than count, average, etc) and as
it is a single digit you want to the sum to include any hidden values.
Function_num
(includes hidden values) Function_num
(ignores hidden values) Function
1 101 AVERAGE
2 102 COUNT
3 103 COUNTA
4 104 MAX
5 105 MIN
6 106 PRODUCT
7 107 STDEV
8 108 STDEVP
9 109 SUM
10 110 VAR
11 111 VARP
best wishes