syafiq - Haven't found much on the community, but if you go to studio, in Text Editor > Email Text and open any email template, down below you will see the available API reference. They basically use Velocity scripting, so it is their documentation which may be of use.
FYI
Email Template API Reference
Customizing email template text and logic is generally considered an advanced task. Programming experience and a familiarity with programming languages is required. The documentation provided here is intended for developers. Non-technical users might find the Email Template Variables table useful to understand information displayed and used within the template.
Email templates are compiled using Apache Velocity. Each template is provided with a unique context that can be accessed programmatically within the email template. See the Velocity User Guide to learn how to use the provided context in your email templates.
Example Variable
In this example, we use the top-level user object to access the nested ranking object, and finally access the getName() method using the shorthand notation: name.
