Native Tokens vs ERC Tokens
A native token pays for a blockchain’s own fees (ETH on Ethereum); ERC tokens are contract-based assets that ride on top and need the native token for gas.
In depth
If you hold USDT on Ethereum but zero ETH, you cannot move it — the network fee must be paid in ETH. The same pattern applies on other chains: BNB for BEP-20 tokens, TRX for TRC-20 tokens. Always keep a little of the native asset alongside your tokens.
Why it matters
It explains the most common "why can’t I withdraw my tokens" support question.