Nikko Pass All Area on Limited Express Revaty

Departing from Aizu-Tajima station, I will be boarding the limited express train Revaty (seats paid) and alight at Shimo-Imaichi station. Can I use the Nikko Pass All Area for the basic fare that covers the section between Shin-Fujiwara station and Shimo-Imaichi station and purchase a separate ticket for the section between Aizu-Tajima station and Shin-Fujiwara station?

Yes, your understanding is almost correct. The only fee covered by the pass is the base fare between Shimo-Imaichi and Shin-Fujiwara. You will have to pay the base fare between Shin-Fukiwara and Aizu-Tajima. But as for the limited express fee, you can buy a single supplement ticket all the way from Aizu-Tajima to Shimo-Imaichi (you don't need two separate tickets).

Thanks for the reply.