The object and host list on which we want to operate is built with +F/-F and +H/-H. With +H/-H we add/delete hosts or groups to the hosts list in the same order they are entered. For example, the expression:
+H dnsservers solaris -H deimos
will result in the host list ``fobos mercurio sar'' since PICA will add the members in groups dnsservers and solaris and delete host deimos3. Note that you can specify hosts not defined in hosts.conf. Those hosts will belong to no group, and connections will be made to that name. Moreover, only ``common'' objects will be seen. But it's a start.
One more feature you will want to know of is the ``automatic exclusion''. Before doing the actual host count, an implicit -H sysdown is added at the end of the list. This allows the sysadmin to automatically delete some hosts that can be down at a given time.
The object list is built the same way using +F/-F, but the object arithmetic is evaluated for every host, since the objects available for every host can be different because we can use conditionals in the objects file. For example, in the objects file above, all documentation objects will only be seen by members of the group ``doc''.
Since we always need at least one host and object, options +F and +H are mandatory. If after doing the host/object arithmetic PICA gets an empty list (either objects or groups) it gives an error message and aborts execution.