If you place down the DAQmx Timing VI on a block diagram, right click on it and select help you will get a list of all the inputs (i.e. rate, source, samples per channel, etc) and a description for each. For example the samples per channel input says "samples per channel specifies the number of samples to acquire
or generate for each channel in the task if sample mode is
Finite Samples. If sample mode is
Continuous Samples, NI-DAQmx uses this value to determine
the buffer size."
I am not familiar with the example you are mentioning, however if you look at the DAQmx shipping example located in NI Example Finder under Hardware Input and Output >> Digital Generation you will find an example called Cont Write Dig Port-Ext Clk.vi. This example shows you everything you need to configure with the digital output.
In this shipping example you need to specify the clock source and the rate. They are both inputs to the DAQmx Timing VI.
Regards,
Jesse O. Applications Engineering National Instruments