A subtask is one of four types, each with its own behavior and specific properties.
An actual task is the core task: a task for which you want to register the actual time you spend on it. These subtasks can be 'In progress'. They are marked with a stopwatch icon.
Task with default duration
For this type of task, you want to register time that can be billed afterwards, but not necessarily the actually measured time, often because it’s about small tasks like sending a mail or filing documents. These subtasks can be ‘In progress’ also. They are marked with an hourglass icon.
For this type of subtask, a mail needs to be sent to the relation, for example for requesting documents. They are marked with a mail icon. When you drag this subtask to ‘In progress’, a dialog opens from where you can send the mail. The text that is suggested in the mail dialog comes from the template that is linked to this subtask, but you can edit before sending. If the mail is sent, the subtask is put in ‘Done’, if the mail is cancelled, the subtask is returned to ‘In progress’, so these subtasks can’t remain ‘In progress’. In the dialog with the details of the subtask, you can consult the mail(s) sent previously and send it again.
A condition is marked with a question mark icon. These are not really tasks, but steps in the workflow that describe a state, often a prerequisite for the rest of the work. This way, you can request documents and then indicate that they have arrived. You cannot register time on these subtasks, so they can’t be ‘In progress’.