For each sync item you can define a list of groups it belongs to from its settings panel. The popup button on the right side of the toolbar shows a list of all groups that is automatically updated whenever a sync item’s groups are modified and is meant as a way to filter sync items.
For example, groups could be used to separate sync items based on their destination device, so that when connecting that device you can select the corresponding group and select the sync items all at once. Another possible scenario is when you want to distinguish between sync items needed at home or at your work place.
The status bar menu also contains an item “Sync group” which will run all sync items in the selected group.