You can have up to 5000 groups per library
You can have up to 5000 group sets per library
The same references can be in multiple groups - deleting from a group does not delete that reference from your main library - however deleting a reference from your main library will also delete it from a group.
N.B. deleting a reference from a smart group will also delete it from the library.
To find out which group or groups a reference has been added to - select that reference in your library then click on References (from the top menu) - Record Summary.
Groups can only be organised under Group Sets (headings) at one sublevel - if you need a further subset of a group you need to create a Group Set and then create groups under that.
The EndNote groups function can be useful when organising references for systematic reviews. The following article explains one possible approach in detail:
Micah D. J. Peters (2017) Managing and Coding References for Systematic
Reviews and Scoping Reviews in EndNote, Medical Reference Services Quarterly, 36:1, 19-31,
Creating groups provides a way for you to organise your references by creating subsets of your library references.
Permanent Groups include All References, Unfiled, and Trash. These are groups that you cannot remove or rename. All References displays every reference in the library. Unfiled displays those references that are not part of a group. Trash is a holding place for references you have removed from the library but have not yet permanently discarded by emptying the Trash.
Temporary Groups include Copied References, Duplicate References, Imported References, Search Results, etc. These temporary groups may be replaced as you use commands in EndNote, and are always deleted when you close a library.
Custom Groups are manually created by the user to help organise the library - you can drag-and-drop to copy individual references into a custom group.
Smart Groups use search criteria to dynamically update groups as existing references are edited or new references are added to the library.
N.B. Deleting a reference from this group will also delete it from the main library.
Combination Groups combine custom and smart groups under a single group to better organise your EndNote references. Use AND, OR, and NOT logic to create new combined group sets. Save references in custom and smart groups, and then save these sets of groups as a combined group set - click on Groups - Create from groups.
Group sets are headings - groups are created/placed under Group Sets - any type of group can be created under Group Sets. References can only be transferred to a Group NOT a Group Set.
My Groups is the default Group Set (Heading) but you can create more Group Sets - click on Groups - Create Group Set in the top menu.
Expand or collapse the list under a group set heading by clicking the plus (+) or minus (-) icon. When a set is collapsed, the number next to the group set heading reflects the total number of references found in all groups in the set.