What's the actual difference between AWS and 13th month?

In practice AWS (Annual Wage Supplement) and 13th month are the same thing, one extra month of basic salary paid usually in Dec. The naming just differs by company. Important part: neither is required by law. AWS is only compulsory if it's written into your employment contract or collective agreement. So read your contract. If it says "AWS subject to company performance" then in a bad year they can legally cut or skip it. Mine got halved during covid and it was fully within their rights because of that clause. Variable bonus is a separate thing on top, tied to your performance and company results, and that one is almost never guaranteed. So your total year-end could be AWS (1 month) plus variable (0 to several months). Check the exact wording before you count the money.

Same thing, different label. The one thing to watch: some contracts prorate AWS if you join or leave partway through the year, and many require you to still be employed on the payout date. So if you resign in November, you often forfeit the whole thing. Time your exit accordingly.

Just to add: AWS is based on your basic salary, not gross, so allowances and OT don't count towards it. Also note CPF is payable on AWS/bonus up to the additional wage ceiling. So your take-home from the 13th month is less than a full month of net pay because CPF gets deducted. First bonus I got I was confused why it was smaller than expected, that was why.

Rule of thumb: treat AWS as fairly reliable if your contract says so and the company is stable, but never count variable bonus until it hits your account. I've seen too many people commit to big-ticket purchases in Nov assuming a fat bonus, then it got cut. Budget on base salary, treat bonus as gravy.