What happens to someone who starts my order form but never pays?

The two-step order form is what causes this

By default, an order form asks for payment details as part of one page. The two-step order form is a separate element you drag onto that same page, and it splits the visitor's journey into two stages: their contact information first, then payment.

systeme.io states this plainly, twice in the same article. The visitor's contact details are added to your list once they finish step one, even if they never finish paying.

So an abandoned checkout is still a lead

Nobody has to buy anything for this to happen. Reaching the payment step and leaving is enough, because the contact was already saved a step earlier.

systeme.io does not use the phrase cart abandonment anywhere in this article, and it hands you no ready-made recovery sequence. What it gives you is the contact itself, and a way to act on the moment they arrive.

Telling them apart from a paying customer

On the order form step, click Automation rules, then Add rule, and choose Funnel step form subscribed as the trigger. Point the action at something useful, such as assigning a tag, and every contact who reaches step one gets it, whether or not they go on to pay.

That tag is what separates someone who only completed step one from someone who completed both.

What a standard, one-step order form does

Nothing is documented here. Neither article describing the order form says whether anything typed into an ordinary, single-step order form is kept if the visitor leaves before paying. The two-step element is the only documented route to capturing that person as a contact.