For my purposes here, I won't draw a distinction between tasks at sea level or below. I'll call them all tasks and you'll know that they have sub-goal levels and they can be combined and recombined in a variety of different ways to meet different goals depending on their context.

However, I will use a different name to refer to things people do that are summary level and above. Here I'll use the term activity.

Examples of activities might be playing a chess game, going for a walk, planning meetings, managing email, or handling incoming customer service calls.

It will take a number of tasks collected together to allow me to participate in an activity. In any given activity instance I might use only some of the tasks depending on the specific details of this particular instance of the activity. Each time I engage in this activity it might go a little bit different.

At any given time I may be engaged in many different activities including multiple instances of the same activity. For example I may be in the process of planning several meetings as part of my regular meeting planning activity.

The very same tasks may find themselves supporting multiple activities. For example looking up an email address may support an activity of managing email, and an activity of planning meetings. Both those activities use my contacts and their email addresses.