A method, comprising: receiving, by a service apparatus, tag information in a network call to the service apparatus, the tag information specifying a given client identifier that (i) is assigned to a user by a publisher that differs from the service apparatus and (ii) represents the user in a publisher domain of the publisher; obtaining, by the service apparatus, a given service identifier that (i) is assigned to the user by the service apparatus and (ii) represents the user in a service domain of the service apparatus, wherein the service domain differs from the publisher domain; creating, by the service apparatus and in a data structure, a mapping of the given service identifier to the given client identifier; generating, based on the mapping, a list of service identifiers corresponding to client identifiers that represent users that have been selected to receive user-specific information; receiving, by the service apparatus, multiple content requests from multiple different client devices accessing services provided by the service apparatus in the service domain; and responding to the content requests based on whether the client devices provide service identifiers that are included in the list of service identifiers, including: transmitting first content reserved for content requests from client devices that provide service identifiers that are included in the list of service identifiers when the content request includes a service identifier that is included in the list of service identifiers; and transmitting second content that is not reserved for the content requests from client devices that provide service identifiers that are included in the list of service identifiers when the content request does not include a service identifier that is included in the list of service identifiers. receiving, by a service apparatus, tag information in a network call to the service apparatus, the tag information specifying a given client identifier that (i) is assigned to a user by a publisher that differs from the service apparatus and (ii) represents the user in a publisher domain of the publisher; obtaining, by the service apparatus, a given service identifier that (i) is assigned to the user by the service apparatus and (ii) represents the user in a service domain of the service apparatus, wherein the service domain differs from the publisher domain; creating, by the service apparatus and in a data structure, a mapping of the given service identifier to the given client identifier; generating, based on the mapping, a list of service identifiers corresponding to client identifiers that represent users that have been selected to receive user-specific information; receiving, by the service apparatus, multiple content requests from multiple different client devices accessing services provided by the service apparatus in the service domain; and responding to the content requests based on whether the client devices provide service identifiers that are included in the list of service identifiers, including: transmitting first content reserved for content requests from client devices that provide service identifiers that are included in the list of service identifiers when the content request includes a service identifier that is included in the list of service identifiers; and transmitting second content that is not reserved for the content requests from client devices that provide service identifiers that are included in the list of service identifiers when the content request does not include a service identifier that is included in the list of service identifiers. transmitting first content reserved for content requests from client devices that provide service identifiers that are included in the list of service identifiers when the content request includes a service identifier that is included in the list of service identifiers; and transmitting second content that is not reserved for the content requests from client devices that provide service identifiers that are included in the list of service identifiers when the content request does not include a service identifier that is included in the list of service identifiers.