The selector role allows you to create a service that selects to which other service a request should be sent to.
Heres an example of how to do that with the help of Perlbal::Plugin::VHosts:
CREATE SERVICE selector_service
SET listen = 0.0.0.0:80
SET role = selector
SET plugins = vhosts
VHOST *.site1.com = service1
VHOST *.site2.com = service2
In this case, requests to both site1 and site2 are received by selector_service, which then sends requests for site1 to service1 and requests for site2 to service2.
Depending on your needs, you may want to use an existing plugin (e.g., Perlbal::Plugin::Vhosts, Perlbal::Plugin::VPaths) or create your own (see selector under Perlbal::Manual::Plugins).