During your online check-in, you have to provide your identity information (passport or identity card) as well as your payment details. The online check-in is secured and your informations are not accessible to any third party.
Your passport/identity card has to be stored by Smartflats during the total length of your stay. This is a legal requirement per the Belgian law (more info here)
Your credit card details are stored until few days after your stay in case of extra charges (extra cleaning, damages, extended stay, etc.)