Here's an overview of what a user can do with an Analytics segment:
1. Create a segment but NOT share or publish it - in this case, the segment will only be visible to yourself and you can use this in workspace.
2. Create a segment, share it with users, but NOT publish it - similar to above, the segment will only be visible to yourself and the user you have shared it with. Both you and the shared users will be able to use the segment in workspace.
In both cases above, the segment will not be visible in any other products - Audience Manager, Target, Campaign.
3. Create a segment and publish it - Publishing an Adobe Analytics segment to the Experience Cloud lets you use the segment for marketing activity in Audience Manager and in other activation channels, including Adobe’s Advertising Cloud, Target and Campaign. There are several pre-requisites to doing this as shared here - https://experienceleague.adobe.com/docs/analytics/components/segmentation/segmentation-workflow/seg-...
Hope this clarifies?