This is the 403rd most frequent Japanese word.
級
"Rank," "level," or "grade."
Here, '上級' (Advanced level) describes the proficiency level targeted by the test.
この試験はとても難しくて、上級者向けです。
This test is very difficult and tailored for advanced learners.
'中級' (Intermediate level) indicates a rank or stage in a hierarchical progression of skill levels.
彼は初心者から中級に進みました。
He progressed from novice to intermediate level.
In this sentence, '一級' (first-class) is used metaphorically to express the highest quality or rank of seating.