The party's address, and the coordinate Katia uses to prove the floor has a second storey.
Station 59 on the Zomp Line stays the party's base while they sleep, reset buffs and rebuild the crafting room. It also becomes the worked example of Katia Grim's theory: dig down from here, she says, and you arrive at another station 59 — upside down, on whatever line carries zomp's inverted colour — with the The Space In-Between sandwiched between them. Carl asks her to confirm that this is literal, and she says it could be right side up, but she does not think so.
The party's last fourth-floor base and the reference point for the mirrored-tangle theory.