The targets web service returns a list of targets for the given metric. Targets can be filtered by station, network, etc. The list can then be provided as a constraint when calling the measurements service.
Supplemental Information for Targets Service
Primary information in Measurements Service help
Much of the necessary supplemental patterns and examples can be found in the Measurements Service help page. The targets service is fairly limited in the parameters it uses, but there is an example for using the targets service to generate a SNCL list file that can then be fed to the measurements service. Please refer to the command line usage section there.