JSE auction process
The JSE has two daily auctions for the market; an opening auction from 8:35 to 9:00 and a closing auction from 16:50 to 17:00. They both work exactly the same and were introduced to enable a transparent opening and closing price for every share each day.
The process is fairly simple. During the auction process, no matching of orders is done. If you submit a buy order at a price that there is a sell order for, the order does not match until the close of the auction period.
All orders can be cancelled or amended during the auction process. At the end of each auction, the system determines at what price most shares will trade. The trade is then executed and that is the opening or closing price.
An important point is that you may actually get a better price then you requested. For example if you submitted an order to buy shares at say R11.50 and the closing price is actually R11.40, you would get the shares at the cheaper (R11.40) price.