Hi Jason & welcome to the boards.
TL is a trade line. Each individual entry for each separate credit account on your credit report is a tl. Your student loans will have an individual tl for each loan you took out, even if they are in the same year.
An example would be if you took out a Stafford subsidized student loan, a Stafford unsubsidized student loan and a Perkins loan in your freshman year, they will each have 3 tl's on your credit report as well as NSLDS.
When you rehab, check NSLDS to make sure all your defaulted loans are included in rehab. You may have loans with different lenders or collection agencies.
www.NSLDS.ed.gov