A seed face occurs in the context of flow volume extractions. If you are not yet familiar with the extraction of flow volume using SimScale you can look it up here.
A seed face is the source face for the flow volume extraction. It is important to choose a suitable seed face for creating an open inner region. All openings of the region need to be assigned to a boundary surface (light blue). The seed face needs to be adjacent to one of the opening faces.
Choosing a seed face can be trickier at times. We advise you to watch our YouTube video for additional examples.
In the context of closed inner region, you need to assign only the seed face and it can be any surface inside the CAD model.
For enclosures just assign any face belonging to the CAD model as seed face.