When visibility at Amsterdam Airport Schiphol is deteriorating below certain limits, the overall ATC capacity is reduced to guarantee safety.
ATC low visibility procedures at Schiphol are categorised in four phases: A, B, C, and D. These phases are based on Runway Visual Range values - RVR - and ceiling. During all four phases all runway entries and crossings are safeguarded by switchable or fixed stop bars.
ATC uses two terms to indicate poor visibility operations:
Although stated in the AIP some pilots are not familiar with the term reduced visibility. This video explains the difference between reduced visibility and low visibility.
An animation: