Hi,
Portal internal target will be created for each policy published on software portal after first user request this policy from software portal (all future requests of this policy from software portal will add computers to this target). To assign same policy to different computers you should create new target.
If you want to remove some computers from this policy target you can open it from the policy and remove resources by using arrows buttons (after removing you not need to click on save as button, it will be autosaved after closing the Resource selector window by click on Ok button):
Regards,
Dmitri