Agent tags are only issued by dealers. It looks like dealers hire agents to buy cars, and the agent tag allows them to transport the car back to the dealership. I could be wrong, but that's what I found in my 15 seconds of googling.
CHAPTER 4. COMPLIANCE & DEALER OPERATIONS4
4.22 Dealer Agents. Dealers are responsible for the actions of their employees and agents.
Under the Dealer Agent Rule, a dealer must give anyone dealing with his employees or agents in a
wholesale situation, a letter of written authority of that agent. The dealer’s authorization will be
valid until either the termination of that dealer’s license or until the dealer revokes the authority in
writing. Once a dealer gives such written authority, the agent may buy and sell vehicles at auctions
and to other dealers in the name and under the auspices of the dealer’s license. The Dealer Agent
Rule found at 43 TAC §215.148, sets out what is required in the letter of authority. Some wholesale
auctions have their own forms for agent authorization. These forms will take the place of the
required letter on dealership letterhead if the form contains all the requirements of the rule.
Under the Dealer Agent Rule, an agent may not pay for a vehicle in cash as all transactions
must be in the name of the dealer using the dealership checks, drafts through the dealership
financial entity, or cashier’s checks drawn on the dealership accounts. Further, auctions and other
dealers may not give the agent the title to vehicles, but must deliver the titles to the dealer at his
dealership