I have my Unication set up to scan a wide area TRS with several sites. I devised a site preference system that works well for me.
I have one system programmed normally -- all control channels and sites inputted in their respective lists.
I have several other systems with only the control channels of the site I want to listen to, and only that site in the list of site aliases. This allows me to only listen to one site of the system at a time. The radio will not connect to another site if it goes out of range. It will only listen to that one site.
If I recall correctly, you can also program only the control channels for your "preferred" site in the CC list, but still input all the site HEX/DEC values in the site alias list. This will cause the radio to try to connect to your favorite site first. With full spectrum scan on, if it goes out of range, it will select ANY site in range.
Please note that you must turn full spectrum scan on in order for the radio to reconnect to the system if it goes out of range no matter the configuration. This issue has been discussed previously on this forum, and arose with PPS 4.0. Not sure if it has been fixed.
I will go back to my laptop sometime and look at the PPS to make sure the above is correct - if there are any corrections I will update this post.