In the book "Cattle Lords and Clansmen", they mention fosterage and what each child of what class should be fed. But, only the top 3 or 4 classes are mentioned. It could be that the authors didn't see fit to list all the classes, or it wasn't all written down by the scribes, but it could also be that the lower classes didn't foster out their children.

This is JMO based on my reading, but I think fostering was mainly an upper class practice. You needed to pay for a child to be fostered. In the case of fostering, the foster parent was a family member from the mother's side of the family, often her brother. In the book, it outlines reasons for this including the safety of the children from competing males from the same family looking to eliminate the competition. I don't think it's a coincidence that the treatment for hostages worked along the same lines as treatment for foster children. Hostages were children of leaders of subject tribes given to the dominating tribe as a type of insurance. The host, just like a foster parent, was required to treat a hostage according to their status.

The lower classes seem to fall into different categories. Could have been slaves or types of laborers. They didn't have use of the land and their inheritance would have been incredibly small (if it existed at all) so that the need for fostering just wouldn't appear to be necessary.