This isn't a rights issue. That's different.
It is the right to receive your dividends in the form of scrip. It is called the Santander Scrip Dividend Scheme. There is no cost to it. It doesn't dilute anyone because you are either buying more stock at the prevailing price or just taking it in cash (and you could use this cash to buy more stock at the same price, for example...plus brokerage... just to demonstrate its equivalence). Essentially, you are going to get a dividend. You then decide whether you want that paid in the form of stock or cash. That's all you need to know. In Australia it would be called a Dividend Reinvestment Plan. If you want the cash, tick the cash box. Otherwise reinvest it to whatever percentage you want.