Recordings and Expiration Dates

Please help me understand why my recordings have an expiration date. I have read here that VOD recordings have an expiration date, and that anything recorded via satellite does not. I am recording seasons of shows, and they are recording on the day they first air, so why do they have an expiration date? Are they not recording via satellite? And if not, how do I record shows that I watch with the option of keeping them until I can watch them?