Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, that enable clients to create custom conditional segments on which other clients can claim a spot. In one aspect, a front-end server receives, over a data communication network and from an electronic device of a creator, one or more data packets that specify attributes of a segment created by the creator through a graphical interface presented at the electronic device. A back-end server detects, based on data received from the device, interaction with an interface control that designates the segment as a conditional segment and, in response obtains, from the device, an expiration time specifying how long the conditional segment is available to others. After obtaining the expiration time, clients are notified that the conditional segment is available and enabled to claim a spot on the conditional segment through a client-side application.