Maps, or tilesets, are the physical locations where you fight in Dreadnought. They are divided in two categories, Atmospheric or Orbital.
Orbital maps are located above orbit, in actual space. In these maps, combat is more positional and up & down have more meaning. There are currently two orbital maps, Sinley Bay and Rings of Saturn. Sinley Bay is only accessible in the Tutorial, though. Ryugu Haven was added in April of 2017.
Atmospheric maps are located a few thousand meters above the actual planetary surface. In these maps, down has a hard limit called ground. You can go very high, but there is also a limit (skybox). Therefore, positioning is more relatable to an orthogonal plane, and up & down mean only cover or open air.