Essentially a process to determine that you are who you say you are (the name you say is yours is actually yours). They did some of that on March/April, I think, but hadnt heard of any cases recently. It is usually not a problem if you have a week or maybe a bit more) after the interview - except for the hassle of going back to collect your papers a week after your interview. But, a huge headache if you fly back to US within a few days of the interview.