Face templates, and the selfie used to query them, are biometric data. Article 7 of the Law of Ukraine "On Personal Data Protection" prohibits processing them as a rule, and permits it only on the grounds that Article lists.
The ground we rely on is your unambiguous consent under Article 7(2)(1). At a closed event the organizer asks for it when you register, before photos are uploaded, because faces in photos are turned into face templates soon after upload, before anyone searches. Before you search, the event page asks you again: the search buttons stay locked until you tick the box, and our server refuses a search without that confirmation. It records each confirmation (the event, the time and the terms version) with no personal data. Anyone who has not agreed can remove their face from search with "Don't index me", even before the event. You may withdraw consent at any time.
The Platform does not record or check the consent of the people shown in photographs. At a closed event it creates a face template for every face in every uploaded photo, and the opt-out removes a face afterwards. Getting consent from those people is the organizer's job.
The organizer, as controller, is responsible for obtaining that consent and for being able to demonstrate it. Biometric data is also processing of "particular risk" under Article 9, so where that obligation applies it is the organizer who notifies the Ukrainian Parliament Commissioner for Human Rights. We set this out in the agreement we conclude with each organizer.
Each event has one of two types, chosen by the organizer when it is created. At a closed event, attendees agree to face search when they register, usually with a checkbox. At an event without biometrics, faces are not analysed at all: there is no face search and only the photographs are stored.
For non-sensitive operational data we rely on our legitimate interest in running and protecting the service (Article 11(1)(6)) and, for organizer accounts, on our contract with the organizer (Article 11(1)(3)).