The green in the image above is the mask surrounding the component solder lands
When encapsulating a high voltage board, it is sometimes desirable that the board be free from mask. A Custom solder mask can be created to surround the component lands only, this eliminates the need for the tedious process of adding discrete solder dams to prevent solder migration during the assembly reflow process.
Designing high voltage boards requires additional clearance, as the voltage increases, so does the spacing requirements. Internal layers do not require as much clearance as external layers. Slots can be employed in order to allow PCB features to be closer and withstand the high potentials.