Clarifying is used when you need to get more information before you can proceed.
It can also be used when you need to nail down a particular part or point or the query.
Examples of clarifying questions:
The latest statistic here is from 2012. Is that recent enough?
What format would like this book in, large print, eBook, or book on CD?
Does the picture need to be in color or will black and white do?
Clarifying may be necessary at some point in the reference interview to clear up specific aspects of a patron's request.