An allowlist is a transfer rule enforced at the token level: addresses on the list may hold or receive the token, addresses off it cannot, and a transfer that violates the rule reverts on-chain. Regulated issuers use allowlists to keep tokenized assets inside their eligibility perimeter.
The important property is where the rule lives: not in each venue's terms of service but in the token contract itself, checked on every transfer by anyone. A tokenized stock that says U.S. persons may not hold can actually enforce it at the protocol layer.
Tokenized assets on U.CASH carry their issuers' on-chain transfer gates, and checkouts surface the live gate status beside your receiving address so a blocked transfer never costs gas.
Every term behind the rails, the assets, and the settlement.