Open almost any task manager and try to break a task down. You will usually find that a subtask is just a task again. It can have its own due date, its own priority, its own reminder, its own subtasks under it. It looks powerful. It is actually a trap.
The moment a step inside your task can hold everything a task can hold, you stop doing the task and start administering it. It is easy to give the subtask a date, then a priority, then another layer down. An hour later you have a beautiful tree of work, and getting started feels more complicated than the task ever was. At this point, planning feels more like progress than actually starting.
Tunl is built on the opposite bet. It is not a to-do list, it is a decide-and-execute engine. Its job is to get you to pick one thing and finish it, not to give you somewhere to file infinite intentions. That bet shows up in a lot of small design calls, and one of the clearest is what a subtask is allowed to be.
What a subtask is in Tunl
A subtask is a line with a checkbox. You can write it, check it off, reorder it, and now, edit its text. That is the whole feature. It does not get its own due date. It does not get its own priority or "fire" flag. It does not get assignees. It does not nest. It is a checklist that lives under one real task, the same way a grocery list or a packing list is a checklist: a flat set of things to tick off, nothing more.
Why we drew the line there
Every field we leave off a subtask is a field that would quietly turn a step into a project. Give a subtask a due date and you now have two deadlines competing inside one task. Give it a priority and you are ranking the parts of a thing you already decided to do. Let subtasks nest and you get a tree with no bottom, which is the exact shape of work that never ships.
When a subtask is really a task
Sometimes the thing you typed as a subtask genuinely is its own work. It has a real deadline. That is not a subtask, that is a task, and Tunl wants it out on its own where it can be decided on and run through the Tunnel. The fix is not to grow the subtask until it can carry a deadline. The fix is to promote it and let the small checklist go back to being small.
The bigger idea
The design goal was simple: make the small thing effortless, and make growing it into a project a deliberate, decisive choice. A subtask that can become a full project makes over-planning easy. A subtask that stays a checklist makes doing the work the path of least resistance. You jot the handful of things you don't want to forget. You check them off. You move on.