Why Excluding a Group from Tagging Can Be Useful
DEVONthink and DEVONnote treat groups and tags, basically, the same but display them different. That means that if Exclude Groups from Tagging is unchecked in a database's properties, the group(s) in which an item is filed also appear as tag(s) attached to it. When assigning tags to an item other groups are proposed as tags while you're typing. If you like this behavior you can quickly file a document in multiple groups and later find them either by their tags or in the respective groups representing the tags.
If you don't like this behavior though, or if you have groups with the very same name appearing in multiple places in your groups hierarchy, e.g. if you have a 'Sent' group inside groups for every year, then you may want DEVONthink and DEVONnote not propose all of them when you type 'Sent' into the tag bar. It becomes hard to decide which one is the one you're looking for. In this case exclude only the 'Sent' groups from tagging.
Or, if you don't like our approach at all and want to go for 'pure' tagging, you can even completely exclude all groups in a database from tagging by checking the respective checkbox in the database's properties (File > Database Properties) and assign tags manually.