Confusion about how to use IC cards for reserved seat tickets on Japanese bullet trains

International travelers and occasional train users in Japan struggle to understand the process of purchasing and using reserved seats on the Tohoku Shinkansen when paying with IC cards (prepaid transit cards). Current documentation is fragmented across multiple sources, and the interaction between IC card payment systems and reserved seat booking procedures is unclear, leaving users uncertain whether they need separate tickets or if their IC card balance covers both fare and reservation.