Validating an Independent Field Value (with FieldVerifying)

Important note: The document you attached is for 2019 - you should get more up to date training materials. I didn’t do T190 but, instead, did the T200 series (T200..T250).

However, in my T220 documentation it reads as you have typed it.  When I look at the supplied code files it shows something different. So I think that the documentation might be incorrect, even in the updated version. My full code listing for that method looks like this:

//Validate that Quantity is greater than or equal to 0 and
//correct the value to the default if the value is less than the default.
protected virtual void _(Events.FieldVerifying<RSSVWorkOrderLabor,
  RSSVWorkOrderLabor.quantity> e)
{
    if (e.Row == null || e.NewValue == null) return;

    if ((decimal)e.NewValue < 0)
    {
        //Throwing an exception to cancel the assignment of the new value to the field
        throw new PXSetPropertyException(Messages.QuantityCannotBeNegative);
    }

    var workOrder = WorkOrders.Current;
    if (workOrder != null)
    {
        //Retrieving the default labor item related to the work order labor
        RSSVLabor labor = SelectFrom<RSSVLabor>.
            Where<RSSVLabor.serviceID.IsEqual<@P.AsInt>.
                And<RSSVLabor.deviceID.IsEqual<@P.AsInt>>.
                And<RSSVLabor.inventoryID.IsEqual<@P.AsInt>>>
            .View.Select(this, workOrder.ServiceID, workOrder.DeviceID, e.Row.InventoryID);
        if (labor != null && (decimal)e.NewValue < labor.Quantity)
        {
            //Correcting the LineQty value
            e.NewValue = labor.Quantity;
            //Raising the ExceptionHandling event for the Quantity field
            //to attach the exception object to the field
            e.Cache.RaiseExceptionHandling<RSSVWorkOrderLabor.quantity>(e.Row, e.NewValue,
                new PXSetPropertyException(Messages.QuantityToSmall, PXErrorLevel.Warning));
        }
    }
}

Have a look for a HelpAndTraining folder within your Acumatica installation - in there you will have the source code for the whole project.