A method, comprising: receiving, by a service apparatus and from a client device, tag information in a network call to the service apparatus that was generated by firing of a tag embedded in a resource of a publisher, the tag information specifying a given publisher identifier for the publisher and a given client identifier assigned to a user of the client device by the publisher; obtaining, from the client device, a given service identifier assigned to the user by the service apparatus; creating, by the service apparatus and in a data structure, a mapping of the given service identifier to the given client identifier; receiving, from the publisher and by way of an API, a list of client identifiers assigned to a set of users by the publisher; determining that the list of client identifiers includes the given client identifier that is mapped to the given service identifier; storing, by the service apparatus, the given service identifier that is mapped to the given client identifier in a list of matched service identifiers based on the determination; receiving, by the service apparatus, multiple content requests from multiple different client devices accessing services provided by the service apparatus; responding to the content requests based on whether the client devices provide service identifiers that are included in the list of matched service identifiers, including: transmitting first content reserved for the list of matched service identifiers when the client device provides a corresponding service identifier that is included in the list of matched service identifiers; transmitting second content that is not reserved for the list of matched service identifiers when the client device does not provide the corresponding service identifier that is included in the list of matched service identifiers. receiving, by a service apparatus and from a client device, tag information in a network call to the service apparatus that was generated by firing of a tag embedded in a resource of a publisher, the tag information specifying a given publisher identifier for the publisher and a given client identifier assigned to a user of the client device by the publisher; obtaining, from the client device, a given service identifier assigned to the user by the service apparatus; creating, by the service apparatus and in a data structure, a mapping of the given service identifier to the given client identifier; receiving, from the publisher and by way of an API, a list of client identifiers assigned to a set of users by the publisher; determining that the list of client identifiers includes the given client identifier that is mapped to the given service identifier; storing, by the service apparatus, the given service identifier that is mapped to the given client identifier in a list of matched service identifiers based on the determination; receiving, by the service apparatus, multiple content requests from multiple different client devices accessing services provided by the service apparatus; responding to the content requests based on whether the client devices provide service identifiers that are included in the list of matched service identifiers, including: transmitting first content reserved for the list of matched service identifiers when the client device provides a corresponding service identifier that is included in the list of matched service identifiers; transmitting second content that is not reserved for the list of matched service identifiers when the client device does not provide the corresponding service identifier that is included in the list of matched service identifiers. transmitting first content reserved for the list of matched service identifiers when the client device provides a corresponding service identifier that is included in the list of matched service identifiers; transmitting second content that is not reserved for the list of matched service identifiers when the client device does not provide the corresponding service identifier that is included in the list of matched service identifiers.