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A group points to a subset of references that already exist in the library. Groups make it easy to break a large library into subsets, helping you arrange materials in ways that are meaningful to you.
The left-hand pane of the library window lists several Group Sets that include various types of groups including Standard Groups, Custom Groups, and Smart Groups. A new library may display only the permanent All References, Unfiled, and Trash Groups. To display a group of references in the Reference List pane select the group name.
Automatically generated groups include All References, Search Results, Transferred References, and Imported References. Custom groups are created by the user to help organize their library, and you can create up to 5,000 groups yourself.
To help you meaningfully organize your groups, you can create custom Group Sets. These can contain any combination of custom groups and smart groups. Use the plus (+) or minus (-) signs next to a Group Set header to expand or collapse the list of groups found in that Group Set. When the list is collapsed, the number next to the Group Set header reflects the total number of references found in each group. This number could include duplicates (references that are included in more than one group).