This is the 2761st most frequent Russian word.
купянском
"Купянском" is the prepositional form of "Купянск," referring to something in or related to Kupiansk, a town in Ukraine.
In this sentence, 'купянском' is used as an adjective describing a style associated with Kupyansk.
Этот храм в купянском стиле был построен в XIX веке.
This church, built in the Kupyansk style, was constructed in the 19th century.
Here, 'купянском' modifies 'районе', indicating that the district is related to the location Kupyansk.
Мы посетили музей в купянском районе прошлым летом.
We visited the museum in the Kupyansk district last summer.
In this instance, 'купянском' describes the project and implies its relation or origin from Kupyansk.