Hey Emma,
Thanks for your post and welcome to the community!
The content section of the invoice is linked to the main Sales settings within the accounts and settings, and so will update and apply across all invoices when this is edited on any template.
Currently the only section on the content tab which allows different text on the invoice is the 'Add payment details and footer' box. You can leave feedback for further customisation options by going to the Gear icon > Feedback within you account. Please let me know if you have further queries :)