There are multiple light source units and single point light sources. Obviously, single point light sources control the light better and it is recommended to be used for a more detailed film exposure. Multiple units scatter light, attacking the films from multiple angles creating “undercutting” that reduce the screens accuracy. Both work, single source works better.
Exposure Unit Vacuum:
A tight seal between the screen, film and the exposure unit glass it critical to prevent “under-cutting”. Under-cutting is when light creeps around the dark areas of the film positive and exposes the emulsion beneath it to light. Frankly, that is a complete failure to the system. Keep your blankets in great shape and achieve the best seal possible.