A universal attribute is your "Custom" parameter. It can contain any data you want. For example, you know what books your subscribers like. You can use that in Soundest. Before you import your subscribers from a file, next to the email addresses, create an additional column and call it Universal. Then enter a genre your subscribers like e.g. John likes "War books", Mary likes "Romance", etc. Once you add all data you want, save the file and import it to Soundest. You can create segments based on Universal attribute or use them to personalize newsletters.
You cannot add multiple attributes (using comma) in Universal attribute at this moment. Use only one attribute per subscriber.
Next to the email addresses create an additional column and call it Universal.