During the intake process, every inmate is assigned a "Segregative classification". This consists of the following:

I  First Offender (21 years of age or less)
IA  First Offender (22-25 years of age)
IB  First Offender (26 years of age or older)
II  Second Offender (21 years of age or less)
IIA  Second Offender (22-25 years of age)
IIB  Second Offender (26 years of age or older)
IIC  Multi-Offender (26 years of age or older)

This helps TDCJ manage the intake process and ultimately send the inmate to the appropriate CID unit.

Once an inmate arrives at their CID unit, they are given a custody classification. There are six levels of classification for non-State Jail offenders in Texas: Administrative Segregation (Ad Seg), and levels 5-1 (G5 - G1) with G1 being the lowest custody classification level (minimum security). There are strict limitations on an inmates housing assignment and work assignments related to their current custody level. For example, G4 inmates must normally live in a cell (not a dorm) and may work outside the permimeter fence only under direct armed supervision (ie a mounted officer with a rifel).

Most inmates are assigned G3 status when they reach their CID unit. An inmate will only be considered for parole if they are at the custody level they were admitted into TDCJ at or lower (less secure). For example, an inmate who was assigned G3 status when they reached their CID unit must be at G3, G2 or G1 if they wish to be considered for parole when they become eligible.

More details can be found here in the TDCJ Offender Orientation Handbook http://www.tdcj.state.tx.us/documents/Offender_Orientation_Handbook_English.pdf