For a bubble type system priority is placed on protecting the product instead of avoiding contamination of adjacent lower cleanliness grade areas. For this purpose, a higher static pressure is maintained in the clean rooms than in the adjacent areas.
With the sink type system, on the other hand, the protection of the environment from the product is emphasized. In order to prevent contaminated air from entering adjacent areas, a negative pressure is established in the hazard zone.
[GMP Compliance Adviser, Chapter 3.C.2]