Does this game account for relative motion? For instance, if an effect depends on someone entering an area, what should that motion be relative to for it to count?
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I'm not entirely certain what you are asking: Generally speaking, areas don't move, so relative movement is irrelevant.
For instance, spells like School Spirit read like "Enemies who begin their turn within the effect or who enter it must succeed at a Will saving throw or be shaken for 1 minute" and play exactly as written: The spell has a distinct non-mobile area (in this case, a 30' spread within close range of the caster at time of casting) and foes need to make a save against its effect whenever they start your turn in this area or you enter this area after the start of their turn. If the area of effect does somehow move, then any foes in the new area would presumably now fall into the 'Enemies who begin their turn within the effect' category.
Could you provide an example of when you think relative motion might come into play?
Precisely that. If the area moved to the enemies, then the enemies entered relative to the area, but not relative to the ground. Likewise, if the enemies are on a moving platform that brings them into the area, they are entering relative to the area and to the rest of the ground, but not relative to the platform. Which matters more? And if the spell is cast by someone on the moving platform, does it stay still? Relative to what? The moving platform? The rest of the ground? The caster?
PF2e doesn't have rules for this either. I thought SF might because space travel.
For playability most spells will be anchored to the game map in use, but I'd allow a caster to specify what reference point the spell is anchored to when casting. So difficult terrain or wall of force type effects could be on the moving platform and moves with the platform.
If the game map was the interior of a grav train, and the players were being pursued by another train, most spells would be connected to the game map, and move with their train. They could set up area of defense spells on their train where they might expect to be boarded if the enemies catch up. However, I'd allow a spell to be cast on the tracks if they can open a back door and have line of sight, and then the pursuing train would then pass through it. If the train is enclosed any damage dealt may be the the vehicle and not the passengers, but if they are on an open platform gliding down the rails they'd definitely be affected.