Every Height task, including subtasks, have their own unique task IDs that are hidden from list views by default.

To find a task ID, you can either open the task or toggle tasks IDs (cmd+shift+I).

On a task page, the ID will be right below its name:

You can also optionally choose to always show task IDs from list views. This is a user-level setting, not a workspace-wide setting, so you can choose what works best for you without affecting what your coworkers see.

To see the IDs next to the task name, you can either turn on "Show task IDs" in Personal settings → Preferences (or use the shortcut cmd+shift+I from anywhere within the app).