Understanding contexts is not about what they are, but what they are not. Contexts do not provide you with the task you are going to do. They are used to filter a long list into a shorter list. From a reduced list one then can make a determination based on time, energy, priority, or any appropriate combination thereof to identify a task to "do".
Since the requirement to further choose a task to do within a context it is always encouraged to write very specific tasks. "Call doctor" will not be clear later when running through a list of calls, nor will "Make doctor's appointment" when you need to email specific information to the receptionist. As a brief example; I would write some variation of the following as tasks.
- Call doctor's office to schedule an appointment next week for my son's annual school physical > context of "iPhone"
- Email doctor's office receptionist with requested forms and verify scheduled appointment date/time > context of "Any Device"
Sample context list:
- Any Device
- Post Office
Before everyone asks: I use "Any Device" and "Anyone" as context under specific parent context to help in making custom perspectives.
My iPhone perspective would be filtered to show only available tasks from a selection of the contexts; iPhone, Any Device, Errands. I can then further choose from a shortened list of what is a priority or something I want to do, like emailing the receptionist at the doctor's office or calling the same receptionist.
My Coworker1 perspective would be filtered to show only available tasks from a selection of the contexts; Coworker1, Anyone. I can then move forward on tasks which can be done specifically with coworker1 or any general tasks that anyone within the office could assist with.
I hope my perspective on contexts helps you in your inquiry.