The new Surface Permeability pack was introduced in Gen 5, specifically to identify whether water is likely to soak into a ground surface or not.
For applications such as flood modelling, storm water management and hard surface estimation, the Surface Permeability pack has superior performance to the "Surfaces" pack. Rather than attempting to distinguish between grass and dirt (where there is much middle ground), or asphalt and concrete (which can be ambiguous in low light or deteriorated settings), the two layers more directly solve the problem. The result is a more contiguous set of layers, with fewer gaps and neater edges than is achieved by taking a spatial union of relevant layers in the Surfaces pack.
When combined with the building footprint pack, the two permeability layers and buildings form a largely complete but non overlapping data set, as depicted below.
Natural Pervious (green), Hard Surface (blue) and Building Footprint (orange) Gen 5 vector layers in Houston, Texas
The Surface Permeability pack includes two AI Layers: