Messages are sent to the Slack API as JSON objects. The Slack message format is defined in the Slack API Documentation.
You can test advanced message formatting with the Slack Message Builder before bringing your message into the Slack Connector add-on and configuring your template keys.
The JSoft Slack Connectors use the StringTemplate library to integrate data from your Atlassian application into Slack message objects. Please see the StringTemplate Cheat Sheet for an introduction to this syntax and the various features it provides.
Each message has a variety of attributes provided to it by the Slack Connector that can be easily accessed with StringTemplate syntax. Additionally, some StringTemplate attributes may be entire Java objects whose properties can be traversed with the StringTemplate syntax to extract exactly the data you need.
When using object attributes, StringTemplate automatically calls get, is, and toString methods on the object when you specify property names. You can learn which properties are available from Atlassian's object documentation.
Please see the documentation for your connector to find exactly what template attributes are available: