In order to categorising a task, and sharing it with people, you simply need to tag it with this a subject. It makes the task accessible to the people in the according subject(s).
Creating tasks in one Subject
You can create tasks in all subjects you have access to. Simply click on the Tasks tab, and start typing your new task's label. Press Enter on your keyboard to create the task.
Creating tasks in multiple Subjects
You can add subjects to tasks you create from any task lists. Click on the + icon on the far right of the task creation field. Then select the subjects you want to share the new task in, and create your task.
Sharing tasks in Subjects
You can add subjects to any existing task. Simply click on the task to open its taskcard and select the + Share on icon under the task label. Again, select the subjects you want to share the task on from the dropdown menu.
You can also click on the x icon next to a subject's name to stop sharing the task on this subject.