A standard is the set of rules by which a token lives on a network: how it is transferred, how the balance is counted, how wallets see it. TRC20 describes those rules for the TRON blockchain.
Why it is the usual choice for USDT
Because of transfer cost. The same USDT exists on several networks at once, and the cost of sending differs: on TRON it is usually under a dollar, on Ethereum it can reach several tens. On a transfer of a couple of hundred dollars, that difference decides it.
The TRON fee is paid not in USDT itself but in the TRX coin, which is spent on network resources. A wallet holding USDT with a zero TRX balance will not send a transfer, and this is the most common cause of «I have the money but I cannot send it».
What happens if you get the network wrong
TRON and Ethereum addresses look different, and a wallet usually will not let you send to the wrong one. But exchanges and exchange services often show the same currency list for different networks, and picking the wrong line there is easy. Funds sent to the wrong network do not physically disappear, yet recovering them is only possible through the recipient's support, and not always.
So the rule is simple: choose the network before you copy the address, and check it twice.
What the address looks like and how long a transfer takes
A TRON address begins with the letter T and is 34 characters long. No address on another popular network looks like that, which makes it the simplest check before sending.
A transfer reaches the network within seconds and is considered confirmed in about a minute. If more than ten minutes have passed, the network is almost never the reason: more often the transfer was never sent at all or went to a different address. You check this by the transfer ID in a block explorer.
The three most common mistakes
Zero TRX balance. The USDT is in the balance but cannot be sent. The network charges in its own resources, and they need a small TRX buffer, usually the equivalent of a dollar.
The address was copied from an old chat. Exchanges issue addresses for a limited time. An old one may already be unlinked, and the transfer will hang.
The network was chosen on the sender's side but not checked on the recipient's. Both sides must be talking about the same network, since a matching coin name is not enough.
Read more: How to withdraw USDT without losing money on the network fee