int64 SUN — six decimal places. Tighter than Bitcoin (8) or Ethereum (18), but plenty deep for any real payment. SUN is named after Justin Sun, TRON's founder.1 TRX equals 1,000,000 SUN. The SUN is the smallest indivisible unit on TRON — six decimal places, half as deep as Bitcoin's satoshi (which uses eight).
TRON chose 10⁶ at launch, presumably because USDT and many fiat-pegged tokens use 6 decimals already. The trade-off is less granular sub-cent precision than Bitcoin or Ethereum, which matters very little in practice for TRX itself.
milli-TRX is sometimes used informally for amounts around 0.001 TRX (= 1,000 SUN), but it's not a protocol-level unit. Most wallets just show TRX with up to 6 decimal places.
BTC has 10⁸ satoshis per coin, ETH has 10¹⁸ wei per ether, SOL has 10⁹ lamports per SOL, and LTC has 10⁸ litoshis. TRX uses 10⁶ SUN — the lowest decimal count among major chains, but enough to handle any practical amount.