Subscription is the Premium plus money withdrawal option.

When the promo-period runs out, you will be offered a choice of playing for free or getting Subscription.
If you choose the "free game", the Subscription will nevermore be available for you with the current character.

However, you will always be able to get the Premium that is a package of advanced services for the free-players. Premium gives all advantages of the Subscription, except for money withdrawal.

In order to purchase the Subscription, click the Treasure Chest button in the bottom left corner of the Game screen and in the window that opens - select the term of your Subscription and click "Purchase".

Payment "for a month" means payment for 30 calendar days.

If you don't have enough Black Gems - click the "Obtain the BG" button.