First off, feel free to indicate which textbook you are referring to. Second, it would do us all good if we knew the quote from the book to which you are referring. That context could help us see what you may not (and have left out stating). In such a review of the text, you might find the author specifying just what he means by 'sampling situation.' I have not seen this term before, but from a quick Google search it seems to be saying "define the sample with respect to its properties and how it relates to the larger population." In that case, so that the researcher can identify where bias can enter into the problem that distorts the sample values from the population to which we want to make inferences.

In the case of your statistical test, the parameter of interest is the difference in proportions. In this and other cases, the sampling situation, I believe, would be the specification of interesting facts: what are the sample sizes, what are the variances, what are the units of measure, etc. Basically, detail all the information about the situation. Usually when I approach such problems I start it with a column titled "Given" and I specify all the values. In this way, I can have a snapshot of all the important values in my calculations. Others I fill in as a summary table for what I calculated through the solution. All that information, it appears to me, is the "sampling situation."