Spatial layers are a PopcornFX feature that enables the power to issue data queries on volumes that store particle information. To put it most simply, it gives a particle the power to retrieve data from other particles, and enables the power to do a lot of different stuff with that data. Actually, anything you want!
Here's an example of fluid-like motion achieved thanks to the power of spatial layers: these particles, projected on a head mesh, are "aware of each other", they're constantly checking if there's another particle nearby and retrieving this neighbor's current position. Based on this, the particle updates its own position to move away of its closest neighbor, making this "particle spreading" effect possible.