[SpiralOcean]

The purpose of contexts is to filter out all the things you have to do... to things you can do at the moment.

What do I have available to me at the moment:
Computer:
I am at a computer and can work on everything that needs a computer to work at.
Phone:
I have a phone and can work on things that I need to do with a phone.

Granted, we live in a time where most of us spend most time at a computer, so many tasks fall into this category.

The other resource you have available to you is time.
I have 15 min before I need to be somewhere, what tasks can I accomplish in 15 min.

Another resource might me working with someone, usually referred to as agenda.

Bob is in front of me now, is there anything that I need to work on with Bob.

Some of the problem is contexts can overlap.

I am at a desk and have my phone & computer.
Should I email Bob, call Bob or do I need a face to face interaction with Bob.

As was pointed out, if you want to work on one project, or a group of projects, OF allows you to focus on that one project, or folder (group of projects) to work on something.

Having a context like, Design, is too ambiguous.

Maybe one of your tasks is:
Design new logo for client. set in a computer context.