You may see people incarcerated in Texas describe their line and custody class as something like G3/L1. G3 is the custody class, which we discussed last week. The L1 in this example is a line class.
Line class has to do with how one earns good time, which affects the amount of time one actually serves.
Line class ratings range from L3 (lowest, earning no good time) to L1. Most people enter prison at L1.
There are also S line class ratings from S4 to S2. The S is short for SAT, which means State Approved Trusty.
Line class is explained on pages 8 and 9 of the handbook published by TDCJ, but there is also a more simple overview at this Texas Criminal Appeals Blog entry.