Instead of placing your business code in the OnValidate trigger of a field, you can also subscribe to system generated events. For each field, we can subscribe to the OnBeforeValidate event and the OnAfterValidate event. These events are triggered, as their name suggests, before and after the code we place in the OnValidate trigger.
Subscribing to these events can be useful to avoid making modifications to the objects that are shipped by Microsoft so that future updates are easier to implement and merging code is prevented.
More information about events in Dynamics Business Central can be found on MSDN: https://docs.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-events-in-al.
