It is often convenient to combine tasks from different projects and projects from different folders if they share some particular aspect.
For example, location. There are tasks that can only be performed in the office, the garage or in a particular city. Or there are tasks that can be solved only when meeting with a particular person. Aspects like that are called Contexts. If you set a context for a task or a project, it will appear on a dedicated context list.
Other useful use of contexts would be combining tasks and projects that share the same priority. Here is how a “High Priority” context might look.