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
In the context of closed inner region the seed face can be any surface facing the flow.
For enclosures you do not necessarily have to assign a seed face.