A Seed List is a Mailing List that will NOT observe the Email Opt Out preferences of any customers on that mailing list. To create a seed list, select the seed list checkbox in the Mailing List properties.
Seed Lists are used by the E-Mail to This List feature.
Generally, seed lists are used for testing or for a select group of customers who should always get a particular communication, or as a handy way to add a list of staff people or key members into an email.