In the Content configuration tab of a "Subtasks creation" recipe, you can find the settings that let you define what will be copied from your parent issue to its subtasks, and what will be synchronized between the issues.

Fields

This section lets you configure which Jira fields must be copied and synchronized between the source issue and its copies.
These options are configured exactly as the Fields options for regular recipes.

However, one option is specific to "Subtasks creation" recipes : Set value for each subtask.
This option can be reach from the action menu at the right of the "Copy from source issue or set value" column in the Fields Mapping table.

When this option is used, a different value can be assigned to the selected field, for each subtask : 

Comments

This section lets you configure how comments are copied and synchronized between the parent issue and its subtasks.
These options are configured exactly as the Comments options for regular recipes.

Attachments

This section lets you configure how attachments are copied and synchronized between the parent issue and its subtasks.
These options are configured exactly as the Attachments options for regular recipes.