In the classical GTD (book version) David Allen's design assumes a mutually exclusive assignment of contexts so that there can only be exactly one Context for a given task. However in modern adaptations and GTD apps (Todoist, Omnifocus, Things) relax this rule to allow for multiple contexts. This is actually quite useful for tasks that span tools or can be done in more than one setting. Adding the ability to do this would be helpful for filtering and would be consistent with the key test Allen emphasizes: "Does it make it easier to decide what to do next in the real world". (Yes, Tags could be used for this purpose, but I use Tags as the major functional areas of my life rather than Contexts).