Any site visitor that you collect data about and is processed in Google BigQuery during the reporting period is an unique user.
User data is processed in OWOX BI Pipeline (Google Analytics Real-Time Data → Google BigQuery).
Active users are merged into unique users by user id. Only unique users are accounted for each reporting period. Timezone of reporting period is UTC, so day partitioning may be different from your data.
E.g. if several users logged in with the same User ID within the reporting period and have been processed by OWOX BI services, they will be considered a single user.
An exception is made for User IDs that are used to authorise more than 100 different Client IDs within reporting period. Such User IDs are ignored, and unique users are identified by unique Client IDs.