The Group By drop-down list options are Date, Resource, and None. The default is Resource.
Group By Date: Resources chosen from the Resources drop-down list are displayed side by side, below each date that has been selected in the Calendar navigation area.
Group By Resource: Tasks are grouped firstly by the resources selected from the Resources drop-down list, then by the date or dates selected in the Calendar navigation area. This is the default Group By view whenever the schedule is first opened.
Group By None: Dates chosen in the Calendar navigation area are displayed side by side. Tasks for resources chosen from the Resources drop-down list are grouped together, below each Date header.
Further information: