What do these numbers mean in relation to education?

I was looking at a salary chart for school employees in my area, and i came across this sheet:


http://www.floydboe.net/information/PDF/08-09_SalarySchedule.pdf


My question is what do the numbers across the top of the column T-4, T-5, T-6, and L-7mean?What do these numbers mean in relation to education?
It appears to be teaching experience but the best way to find out is to ask at the school district./