What is the 'NotifyWhe​nScansQueu​edBelow' in session based interface?

Hi all,

I've studied and tested ni-daq device.

But I can't understand what is the 'NotifyWhenScansQueuedBelow' feature.

s = daq.createSession('ni');
addAnalogOutputChannel(s,'Dev1',0,'Voltage');
outputData = (linspace(-1,1,1000))';
s.queueOutputData(outputData);

s.NotifyWhenScansQueuedBelow = 500;
lh = s.addlistener('DataRequired', ...
    @(src,event)disp(1));
startForeground(s);

So I tried some code like this to check when the 'NotifyWhenScansQueuedBelow' work.

I thought it would display '1' when NotifyWhenScansQueuedBelow work, but it doesn't happen.

I changed value of 'NotifyWhenScansQueuedBelow', But nothing happened.

Please let me know what is this, and how can I apply this in practice.