A battlespace is a group of sides which have the means and the opportunity to engage each other on the current turn. Battlespaces are fluid, depending on the territory that each side controls, the location and move of their units, and their political alliances, on each turn.
Erfworld uses Natural PredictamancyErf-b0-p27 to determine which sides share a battlespace each turn. Units may infer the presence or absence of other sides in their own side's battlespace based on turn order.
Within each battlespace, the turn order of the sides is determined based on alliances and ages of the sides. Sunset within each battlespace happens when the last side's turn ends within that battlespace.
Separate battlespaces may have sides playing their turns simultaneously (one side at a time per battlespace).
Real World References
The localization of time within each battlespace is quite similar to the localization of causality within a light cone.