You can earn points by making posts on the forum, no BS spam posts though as they will get deleted. You also get points when someone likes what you post.
You can use these points (select use points) to buy subs for Arma 3 where you get an extended loadout.
You can also directly buy points, this works out cheaper than buying a sub as the more points you buy the cheaper they become.
You can buy points using your debit/credit card by selecting Stripe or you can use other methods such as Paysafe card and other pre-paid cards using Paymentwall
Paying through Paymentwall will give you the payment options available to you in your country