User Segments represent a set of users of your app and the means to identify them. For example: all buyers, all users, those who completed the tutorial, and so on.
The major role of segments is to tell TROPHiT how to identify users during redemption, through a process called Identity Lookup. Every voucher you create on TROPHiT is associated to a segment that helps control the identification of users during its redemption. For example, users might be identified by their app-specific player IDs, device IDs or some other means. TROPHiT uses those identities to enforce redemption limits on users. Every segment may define a different way to identify users, however, the default and recommended way is to use app-specific player IDs, also known as the Player Identity Lookup. Learn more about user identities and identity lookup.
The second role of segments is to reserve vouchers to specific user identities, which effectively limits vouchers to specific users who are deemed eligible. Segments can either require or not require reservations. Segments that do not require reservation only help identify the user during redemption, but will not prevent users from redeeming. Segments that do require reservation will not only identify the user, but also check of that user's identity is whitelisted in the segment. For example, a segment of "all users that have made a purchase" should probably include a list of all paying users based on their player ID and require this reservation to exist in order to complete redemption. Otherwise, a voucher that is aimed at paying users might end up being redeemed by non-paying users through some unexpected leak in the marketing distribution channels, etc.
You can create as many segments as you feel are required for your needs. Learn more on how to manage user segments.