TRC-20 and ERC-20 are token standards on the Tron and Ethereum networks; the same USDT exists on both, but a TRC-20 transfer typically confirms in about a minute for cents, while ERC-20 costs an Ethereum gas fee and takes a few minutes.
A token standard defines how a token behaves on a given chain. USDT on Tron follows TRC-20; USDT on Ethereum follows ERC-20. They are not interchangeable: tokens sent to an address on the wrong network are not received, and recovery is not always possible. The choice affects fee, speed and, marginally, which wallets and exchanges support withdrawal.
For paying a service, TRC-20 is usually the cheaper and faster option; ERC-20 makes sense when the funds are already on Ethereum.
At SMSMeteor: the deposit screen states the network for the address it shows; send on that network only. The USDT page explains the choice.