This is the 6168th most frequent Russian word.
таможенного
"Customs" (related to border control or duties).
The adjective "таможенного" (of customs) describes an entity related to customs, such as "таможенном посту" meaning "customs post."
На таможенном посту проверяют документы.
At the customs post, they check documents.
The genitive form "таможенного" is used to describe possession or association in "таможенного инспектора" meaning "the customs inspector."
Таможенного инспектора нужно было спросить о правилах.
It was necessary to ask the customs inspector about the rules.
The prepositional usage "таможенного дела" (of customs operations) relates the studied matter to customs activities.