DAQmxGetReadAvailSampPerChan parameter

For some reason I couldn't find a detail description of this function

DAQmxGetReadAvailSampPerChan, which takes task handles and data

I don't understand what the data parameter means, so I am confused what I should put it here.

Any one help me to understand this function??

What I am trying to do is

while let a NIDAQ board continuously record signals, I want to read predefined numbers of data so that I can process it later.

I think this function make sure the size of data I want to read for each loop.

If another approach would fit better to my purpose, please recommend me it.

Thanks