If you want to make a segment, but not sure how to filter out just the contacts you need, you can add a new value just for this purpose. 

This can be useful, for example, if you want to filter just the contacts starting with a particular letter. 

If you have your list outside Mailigen platform, just add a new column and create a new easily distinguishable value, for example, “1” and import these details using “update existing subscriber details” option. Follow the steps in this video​

After you have added the new value, creating a segment should be easy. 

Follow the steps in this video