While universe is not 2-dimensional, for simplicity star map is represented on 2d plane called starmap. This view does not have any physical meaning and is just for easier representation.
Starmap size is measured in SMU. Stars should never be closer than 100 SMU to each other. Map has a center at 0,0, and stars can have coordinates from -3e9,-3e9 to 3e9, 3e9. Negative coordinates are top left.
SunRays is universe generation algorithm that generates Sol as 0,0 and a branch of stars for each player. Players start from the end of their chain and are incentivised to work towards Sol, as commerce tends to flow through the center.