The sum of the specified gross wages.
Example to clarify: Assume you work in both Germany and France. Your gross salary from Germany is 50.000 Euro and is shown in line 3 of your employment tax statement. In addition, you have tax-free gross wages of 5.000 Euro, which is shown in line 16 of your employment tax statement. You have received 30.000 Euro from your French employer, for which no German wage tax was withheld.
Your subtotal is then: 50.000 Euro (gross salary Germany) + 5.000 Euro (tax-free Germany) + 30.000 Euro (gross salary France) = 85.000 Euro