You can set the subject of emails using either the Magento Admin or a theme.
Set Subject Using the Magento Admin
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Go to Marketing > Communications > Email Templates and click Add New Template.
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Choose the template and click Load Template.
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In Template Subject, add plain text to use as the subject of the emails sent using the template you create.
This field can contain system variables. For example,
{{var store.getFrontendName()}}: New Order #{{var order.increment_id}}
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Click Save Template.
Set Subject Using a Theme
You can set the subject for an email by customizing its HTML file via
<!--@subject {{trans "Your %store_name order confirmation" store_name=$store.getFrontendName()}} @-->