The third line indicates the name of the data file, e.g., "data.dat".
"NIDW" stands for the number of characters used in the ID number, for example here we have ##; hence "NIDW=##". Next, we note that we are going to use BILOG to estimate the 3PL model, thus, "NPARM=3" (number of parameters for each item). Note the spacing and location of each of these values, BILOG is sensitive to violations to this format.