The Users in Segment filter is particular helpful when trying to organize a large segment.
When creating a custom segment, one of the filter features available is "Users in segment" which allows you to nest large sets of rules defined in an existing segment into the segment you are building. This way, you can easily repeat sets of rules across segments, instead of re-creating them every time.
The filter feature looks like this:
This feature is also useful for excluding a group of users who meet a specific set of rules, or users who are not in a specific uploaded list. To exclude users, set the final value for "users in segment" to "false".