Assigning contexts to actions
In addition to keeping actions organized into projects, single-action lists, and folders, you can also assign each action a context, to indicate where you need to be or what you need to have available in order to complete the action.
To create a new context:
Make sure you are in context mode; the view switcher in the toolbar should have the Context icon highlighted: 
Click the plus button at the bottom of the sidebar.
Type a title for your new context.
To assign a context to an action in planning mode:
Click in the Context column for the action.
Type part of the name of the context you want to assign; OmniFocus tries to match what you typed with the names of your existing contexts.
If you see the context you want, select it. Otherwise, you can finish typing the context name and then press Command-Return to create it as a new context.
To assign a context in context mode, just drag an action into the context's entry in the sidebar.
Once your actions have contexts assigned, you can selectively view your actions by context, to see just the ones that are actually available to you wherever you are now.