A group is a set of items with something in common, such as e-mail messages from the same sender or tasks with the same due date. Group items to see related items together. You can expand or collapse the group headings to display or hide the items they contain.
Expand/collapse
buttons on group headings
You can only group items that are in a view based on a table or a timeline view type. When you group items by a field that can contain more than one entry, such as the Categories field, items may appear more than once in the table or timeline. For example, if you group by the Categories field and an item has two categories, such as Business and Ideas, the item is listed under both the Business group heading and the Ideas group heading. Although you see the item more than once, it exists as only one item. Any changes you make to one instance of the item are stored with all instances of the item.