a unit preview involves mostly your future bosses walking you through your work area, giving you a more in-depth understanding of your job as well as the working environment. i for one did not undergo a second interview during the unit preview, but i've heard that some applicants actually had to go through that.

i guess it depends on the position you applied. but i believe that is mostly if the bosses actually hold interest in 2 or more applicants for a particular position. it also gives the applicants a chance to make a final decision on whether they believe they are suitable for the job or not. hope it clarifies!