Subjects are defined for the emails in the schema of the template. You can define the subject in template like this:
{
"subject": "This is the subject for the email. You can use subject line variables like this: ${FIRST_NAME}."
}
You can also localise the subject like this:
{
"subject": {
"en": "Hi ${FIRST_NAME}!",
"fi": "Hei ${FIRST_NAME}!"
}
}