To provide minimum vertical separation while pilots are NOT executing a discretionary descent, how should an altitude be assigned?

Assigning an altitude once the aircraft has left the previously assigned altitude is the best practice in ensuring minimum vertical separation while pilots are not executing a discretionary descent. This approach maintains safe separation between aircraft, as it takes into account that the aircraft is actually in the process of climbing or descending.

When altitude adjustments are made after the aircraft has already started to depart from a prior altitude, controllers can monitor the aircraft's progress and ensure that vertical separation is maintained throughout the transition. This is particularly important in busy airspace where multiple aircraft may be operating at varying altitudes.

In contrast, issuing clearances before the aircraft begins to leave an altitude, or while the aircraft is still holding a prior altitude, could potentially lead to confusion or miscommunication. These scenarios might place aircraft in closer proximity to one another, which could compromise safety standards in vertical separation. Therefore, assigning altitude only after the aircraft has left its current altitude is essential for effective air traffic management and maintaining safe operational standards.