SSON is not a mutual fund - it‘s an exchange-traded investment trust… Or much like an ETF, if you will, but unlike the main „Fundsmith equity fund.
Mutual funds and ETFs, closed-end funds or such investment trusts are, technically, quite different especially in how they‘re bought, sold and „delivered“.
SSON works as any other company‘s shares that can be traded throughout the day with any market participant at market prices on the exchange it‘s listed.
The main Fundsmith equity fund shares however will ultimately be bought and sold from/to the fund.