The Texas Administrative Code is a subject-based compilation of all the regulations promulgated by state agencies in Texas. It is the state equivalent of the Code of Federal Regulations. Every time a Texas agency (such as the Texas Commission on Environmental Quality, the Texas Railroad Commission, or the Texas Department of Transportation, among others) issues a regulation, it is first published in the Texas Register. This is the state equivalent to the Federal Register, though the Texas Register is only published weekly. Once the regulation comes into force, it is then codified into the relevant title of the Texas Administrative Code.
You can find a complete list of Texas agencies at the TRAIL website: https://www.tsl.texas.gov/apps/lrs/agencies/index.html
The TAC contains sixteen subject-based titles. Each regulation is categorized into a title for codification. Note that there are a lot of numbers that do not correspond to a title.
Title 1: Administration
Title 4: Agriculture
Title 7: Banking and Securities
Title 10: Community Development
Title 13: Cultural Resources
Title 16: Economic Regulation
Title 19: Education
Title 22: Examining Boards
Title 25: Health Services
Title 28: Insurance
Title 30: Environmental Quality
Title 31: Natural Resources and Conservation
Title 34: Public Finance
Title 37: Public Safety and Corrections
Title 40: Social Services and Assistance
Title 43: Transportation
Within each title are “parts.” For example, the parts in Title 1: Administration are as follows:
Part 1: Office of the Governor
Part 2: Texas Ethics Commission
Part 3: Office of the Attorney General
Part 4: Office of the Secretary Of State
Part 5: Texas Facilities Commission
Part 7: State Office of Administrative Hearings
Part 8: Texas Judicial Council
Part 10: Department of Information Resources
Part 12: Commission on State Emergency Communications
Part 15: Texas Health and Human Services Commission
Within each part are chapters, within each chapter are subchapters, and within each subchapter are sections. Section citations use the format “§3.25,” where “3” refers to the Chapter and “25” refers to the section. A full TAC citation may look like this: 1 Tex. Admin. Code § 3.25 (2009). The first number is the title, the number after the section symbol is the chapter and section number, and the year establishes which edition of the TAC you are referring to. Parts and subchapters are not used to cite to a TAC section.