In a Cadillac Seville with the 4.6L Northstar engine, the term "half case" typically refers to the
oil pan. More specifically, it might refer to one of the two halves of a *two-piece* oil pan, if such a design were used on that particular year and model. Not all Northstar oil pans are two-piece; many are one-piece.
So, while "half case" isn't standard terminology for this application, the context strongly suggests it's referring to a section of the oil pan. To be certain, you'd need to consult a repair manual specific to your Seville's year and model.
