One potential issue with considering selective conformal coating with a robotic conformal coating system is misaligned components on the PCB which can potentially damage the spray heads which are moving along a set pattern on the board and the components themselves be damaged as they travel. Spray heads are one of the most critical parts on the spray robot. Repairing damage can get costly.
Parylene Coating Blog by Diamond-MT
Technology varies between the platforms and the valves but essentially they achieve the same result. The goal is to deposit coating where you want it, removing the issue of masking which is required in batch processing with dip or spray booths.