So this is a weird, dopey feature that's unique to the Singaporean stock market.
In most countries, your stocks and your cash are held in an account at the broker, so the broker can see what you own, and how much you can buy or sell.
In Singapore, the broker generally doesn't touch your stocks or your cash - the cash gets paid through EPS on the settlement date, and the stocks settle into CPF, which the broker can't really see into. So the broker can't see how much money you have to spend on stocks, and they can't see what stocks you own (so, how much you can sell).
This is pretty dumb, let's be honest! But that's why you have a buy and sell limit - because the broker can't see how much money you have to spend, or how many shares of stock you own, they have to put an arbitrary limit on how much you can buy and sell each day. If the limit wasn't there, you could buy fifty gigazillion dollars' worth of Amalgamated Pennyfarthings LLC and then just not pay up three days later; or, worse, you could sell fifty gigazillion shares of Amalgamated Pennyfarthings LLC, completely tank the company, and then three days later say "um no, I don't have fifty gigazillion shares to deliver, why yes I am currently on a plane to a country with no extradition treaty how did you guess".
This is also known as the O'Hare Spread, because in its original Chicago incarnation you'd buy or sell hundreds of thousands of futures contracts, and at the same time you'd buy a ticket from Chicago O'Hare airport to somewhere like Rio de Janeiro, with nice weather and no extradition treaties with the USA. If the position goes your way, you close it out at a gigantic profit and tear up the plane ticket; if it goes against you, well... you head to the airport and let the clearing firm sort out the mess.