Has mentioned earlier, the relation between task and time is key.
Tasks are classified by looking at their `frequency` and their `opportunity window`. The `frequency` of a task is the rate at which it is repeated. The higher the frequency the more often the task needs to be done. When a task needs to be done, it is said to be `mature`. And the time it takes for a task to mature is the `period` of the task. The `opportunity window` of a task is the time during which the task can be done.

Evaluating how often a task needs to be done can be difficult. The frequency of some tasks is prescribed by a maintenance plan (e.g.: changing the oil of a car). For other task it is subjective and depends on your personal judgment (e.g.: how clean the floor needs to be). To make things harder, no matter if the period is evaluated objectively or subjectively, it may change in time or vary based on the usage. Actually, the usage would be a better criterion of tasks maturity, but evaluating how much a thing is used, or the wear, is not always simple. Using time give a good approximation. In the end, what needs to be defined, is the `natural frequency`: the rate at which a task needs to be done to avoid negative consequences without overspending resources.

By considering the frequency and the windows of opportunity, tasks can be grouped in different categories which are depicted and described below. Having those categories will allow processing each task appropriately.

## Low Frequency (L)

Low Frequency tasks mature slowly (e.g.: over a period of one month or more) and they have a large opportunity window. They often take more time to do and are more complex. They have a slow feedback loop. Because they mature slowly, the consequences of neglecting them take more time to appear.

## High Frequency (H)

High Frequency tasks mature quickly (e.g.: in less than a month) and they have a large opportunity window. They are often short and simple tasks. They have a fast feedback loop: Because they mature quickly, the consequences of neglecting them is quickly visible.

## Routine (R)

Routine tasks mature very quickly (e.g.: once or more per day). They are often short and simple tasks. They have a narrow opportunity window. They have a fast feedback loop: The consequences of neglecting them is quickly visible. They donâ€™t require reminders as they are done almost automatically and are often anchored to other events of the day.

## Scheduled (S)

Scheduled tasks have a very narrow opportunity window. They can mature at any rate but they need to be done at a specific time and date.

Scheduled tasks can be "sticky" or not. A sticky task, if not done in due time, is carried over to the next day with a penalty. For example, paying the credit card bill is scheduled and sticky. If you don't pay on the due date, you still have to pay the next day plus interest. A non-sticky task is not carried over. If not done when scheduled, the opportunity is lost. A good example is taking out the trash.

## Projects (P)

Projects are non-recurrent tasks, they don't have any period. They can be short or long, simple or complex; it does not matter. What matter is that it will be done only once; they are partially or completely novel to the doer. They may, but more often don't, have a due date which will correspond to the end of the opportunity window. They may have preconditions which will shift the opportunity window's beginning (aka: the earliest start time).

## Erratic (E)

Erratic tasks have an unpredictable maturation rate or opportunity window or both. They will happen, but it is unclear when and how long it will take to do them. This system is not concerned with erratic tasks. However, having a system in place helps to handle erratic tasks.

## The Containers

The container's job is to hold reminders of the tasks to do. They are tailored to different types of tasks and how they relate to time. Having the right task in the right container will ensure that no task is forgotten and the most important ones are easily visible. Having task reminders organized in the right containers will reduce confusion and feeling of being either overwhelmed or stagnating.

The first container is the `Upkeep List`. It is dedicated to the recurrent, high and low frequency, tasks. The focus is on the period not the widow of opportunity. Tasks are organized to avoid high frequency tasks starving lower frequency tasks.

The second container is the `Projects List`. It holds the non-recurrent tasks and organize them to prioritize between urgency, importantance and opportunity.

The third container is the `Calendar`. It holds scheduled tasks and other time-sensitive information like due dates or events that may impact your plan without involving you directly.

Selecting the right container for a task follows the next three rules.

1. Scheduled tasks, recurrent or not, go on the `Calendar`.
2. Non-recurrent tasks go on the `Project List`.
3. Recurrent tasks go on the `Upkeep List`.
``````graph LR
START[Task] --> IS_SCHEDULED{Is it scheduled?};
IS_SCHEDULED -->|Yes| SCHEDULE[Calendar];
IS_SCHEDULED -->|No| IS_RECURENT{Is it recurent?};
IS_RECURENT --> |Yes| UPKEEP[Upkeep List];
IS_RECURENT --> |No| PROJECTS[Projects List];``````