When creating a group, an option is available to create a seed list group. This feature allows you to create multiple groups according to your different needs. This is useful if, for example, you are sending personalized communications with very different business rules. You can then create a control group according to specific criteria (e.g.: customer vs. prospect, newsletter subscribers vs. promotion, etc.). 

To be part of a seed list group, the profile must be identified as a seeded contact and meet your criteria. 

Path to follow : à Contact à Group à Create a group

You can also edit a group that is already made in order to made it a seed list group. 

Path to follow : à Contact à Group à Edit