From DGT GTD
A context is one method of breaking down tasks into more managable groups by giving them a label. Unlike Tags, only one context can be assigned to a task at a time, so when deciding on the method to use to seperate the task list, the contexts should be mutually exclusive.
The theory is that the context describes a person, place or tool that is required to complete an action. When consulting the master task list, any task identified with a context that is not currently available to the user can be hidden, resulting in only the list of tasks that can currently be worked on.
Traditionally, in order to assist the user in distinguishing contexts from other types of labels, each individual context would be prefixed with "@".
For an excellent discussion on good (and bad) context lists, see the article at http://www.evomend.net/en/what-not-gtd-context
From the main screen, select the @ sign on the bottom toolbar. This will bring up the list of currently defined contexts. Selecting the Edit button in the top left corner will modify the page to show a "New context..." option at the top of the context list, and will place a horizontal red bar to the left of any previously defined context.
Select the "New context..." option. A text box will be presented, asking for the name of the context to be created. While contexts traditionally start with a "@" sign, this is not a requirement. Enter the name of the context and select the "Save" button in the top right corner. The text box will close, and return to the individual context editing screen.
If you wish to assign a custom color indicator to this context, select the "Color" option. A color picker window will open. Choose the base color using the wheel in the middle, the saturation using the slider on the left, and the lightness using the slider on the right. When the desired color is visible in the center of the circle, select that circle to return the color choice to the context editing screen.
(Note: in order to have contexts display using the chosen colors, the setting "Use default context color" must be unchecked. This setting can be found under "Task list settings" from the main Settings screen.)
When satisfied, select the "Save" button in the top right corner again to create the new context and return to the context list.
To delete a previously defined context, open the context list as described above, and select the Edit button. Find the context you wish to remove, then select the horizontal red bar to the left of the context. The red bar will rotate to become vertical, and a red "Delete" button will appear. To confirm the deletion, select the "Delete" button. To cancel the deletion, select the vertical red bar - this will rotate the bar back into a horizontal bar, and the "Delete" button will be removed.
Any tasks that were marked with the deleted context will have that context removed.
Almost the same process as creating a new context - open the context list and select the "Edit" button. Find the context you wish to edit, and select it anywhere other than the red deletion bar. The individual context editing screen will open, and allow the modification of the context name or color just as when creating a new context.
If a previously defined context is modified, any task referencing that context will automatically show the new name.
When in the "Task Details" screen for a particular task, scroll down until the "Context" option becomes visible, and select it. A popup box will open, presenting a list of currently defined contexts and an additional option for "None".
If the context you wish to use is already in the list, select the appropriate context. If not, select the "Edit" button to be taken directly to the main Contexts screen, as described above. Make changes as necessary, then press the phone's "Back" button to return to the context assign popup and select the newly modified context.
Tasks without any context will be shown in the "Inbox"-list.
Note: be careful not to select the desired context while still in the main contexts screen - this will not assign the context to the task, but will instead filter the main task list to only show that context.
From any screen that shows the bottom toolbar, select the "@" sign to be taken to the context menu. Without going into edit mode, select the appropriate context to automatically filter all lists to only show tasks assigned to that context.
To remove the filter, perform the same steps but select the "All" option at the top of the context list or double-tap the filter strip.
To only show tasks that do not have a context assigned, select the "No context" option.