Does a door frame have to be softwood or hardwood? Can I use either for the door lining?

Provided that the density exceeds 420kg/m3 and the timber is of good quality with minimal defects, then either softwood or hardwood is suitable for FD30 doorsets. For FD60 doorsets a good quality, straight grained hardwood with a density in excess of 650kg/m3 should be used unless the door manufacturer can provide evidence that softwood can be used. In all cases, you should follow the door manufacturer’s detail.