Hi Fajar! :)
For example, the subject field is totally customizable in the screenshot below.
Thinking about helping our users with that, we also have the “smart tags”. That means you can use any header of your spreadsheet as a subject or at least part of them.
In this example, my spreadsheet is simple, containing First Name, E-mail, and Date.
I must send an e-mail based on the date, so:
And the subject has changed :)