It joins the cluster as your own node. Everything you build runs there and nowhere else, and nothing from anyone else is scheduled onto it.
New accounts pay as they go from a balance, so nothing runs that has not already been paid for.
You can switch to monthly billing after 2 days and €20 of top-ups.
Billed monthly: the machine's price plus the €9.00 platform fee. Stop it from your dashboard whenever you like — there is no term, and it keeps running to the end of the month already paid, then it is removed and billing stops.