Templates in Zoho Books are used to send out Estimates, Invoices, Credit notes and Purchase Orders. They can be broadly classified as PDF templates (for your invoices, estimates, etc) and email templates (mail notifications sent to your contacts). They can be created and modified under the Settings section. If you want all transactions relating to a particular contact to contain a unique template, you can match the template with that contact.
The image shows how each contact can be associated to a PDF template and an Email template. Click on the drop down to choose from the templates you’ve configured under the Settings page.
Save your changes to ensure that all future transactions with that contact will be created using these templates.