Sublevel nine existed inside the Galactic Senate Building situated on Coruscant. This area was the location of the structure's sewage treatment center and was a stop on the introductory tours given to freshly chosen senators.
During 28 BBY, Senator Padmé Amidala, in her initial weeks as a senator, expressed her disappointment to her handmaidens regarding her perceived lack of important duties within the Senate. She sadly commented that guiding people to the waste processing plant on sublevel nine was practically the extent of her contributions.
The novel Queen's Shadow, authored by E. K. Johnston and released in 2019, contained the first reference to sublevel nine.