AFAIK, aside from SSSS, none of them mean anything. In fact, more often than not, they've never even looked at again. It's not at all uncommon for screeners to wave you into a specific lane. I was pulled out of the WTMD priority lane at DEN and told by the screener that I had to go two lanes over to an open scanner based on "random selection" of passengers. I just told the lady that my belongings were on this line and in the X-ray, and I was not leaving them that far out of my sight and control. She shrugged and grabbed the guy behind me (the poor sap whose belongings weren't in the X-ray yet) instead.