"Type" refers to both the type of signal (digital or analog) as well as whether it is input or output. For instance, analog input and output cannot be included in the same task. That having been said, it is possible to perform a DAQmx read on a digital output task. This feature can be used to "snoop" the state of the line. Are you using LabVIEW? For examples of specific operations (we have one for most any basic application), please check out the LabVIEW example finder (Help->Find Examples). Under "Hardware Input and Output"->"DAQmx," there should be plenty of examples of valid DAQmx tasks.
Hope this helps,
Product Marketing Engineer