"No pre-set limit" refers to the ability to turn your credit card temporarily into a "charge card", like the Amex of old. If you go over your credit limit, assuming they approve the charge, then that amount is treated as immediately due, and gets added to your normal minimum payment.

The old-style Amex (and Diners) have to be paid off every month in full, and in return they advertise "no pre-set limit". Mastercard came up with this "world mastercard" hybrid scheme to be able to say the same thing.

How much can you spend? As spelled out in an earlier post, it depends on your spending history and your payment history, and wouldn't be more than they expect you could pay back within a month.