AirlineSim is a pay-to-play game that works with in-game credits, which you can buy.
When you found a holding on a game world, a certain amount of credits will be deducted every 24h. If your credits balance turns negative, you won’t be able to play AirlineSim anymore, but your airline will still continue to exist, allowing you to recharge your credits still. For a trial account, the grace period is 7 days and for premium accounts it is 28 days before your airlines will be deleted after your credits balance turned negative.
Each player has a credit account, which starts off loaded with 60 free credits on account creation. This is enough for a two-week trial period, assuming you only run one Holding. If you want to continue playing after your trial is up, or make use of game features which are only open to paid accounts, you’ll have to purchase further credits. The credit system is completely flexible; buying credits does not constitute a repeating subscription, you can buy credits whenever you like, and if you ever decide to stop playing the game all you have to do is let your credits run out. Credits can’t be refunded, though.