A settlement layer finalizes in its own money.

A settlement layer is where value becomes final, in the denomination that layer already uses.

Countries

Japan settles in yen. The United States settles in dollars. The United Kingdom settles in pounds. A shop in Tokyo is paid in yen. A shop in New York is paid in dollars. A shop in London is paid in pounds.

The country is the settlement layer. The currency is the denomination that layer already uses. Yen in a Tokyo account does not settle a bill in New York.

Same rule

Solana

Settles in SOL.

SOL pays the gas and settles the value. YAP/SOL is the starting tender: YAP is the economic unit, and SOL is the settlement example.

Ethereum

Settles in ETH.

Value that finalizes on Ethereum settles in ETH.

Bitcoin

Settles in BTC.

Value that finalizes on Bitcoin settles in BTC.

A quote denominated in USDC settles in USDC, for the same reason a dollar invoice settles in dollars.