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 method includes receiving, from a device of a creator, attributes of a conditional segment created by the creator through an interface presented at the device. In response to receiving the attributes of the conditional segment, clients are notified that the conditional segment is available and the clients are enabled to claim a spot on the conditional segment. A determination is made whether to convert the conditional segment to a confirmed segment based on a number of spots claimed by the clients. Prior to converting the conditional segment to a confirmed segment, a given client is cancelled from the conditional segment in response to receiving, from the given client, a request to cancel the claimed spot.