Route advertisement - Selective???

Hi friends...

Can you tell me what is the meaning of Selective in the route advertisement option in Virtual Address properties? F5 documentation is too vague on that.

Here is my issue, I have several F5s in different areas. All of them have exactly the same configuration(even IP address), and the routers that it is connected also have exactly the same configuration.. however, in some of them, F5 is not advertising that VS in the route table. The VS does not have a monitor so its status is in blue color.

I know to fix that I have to change it to Always, but what Selective means? What F5 take into consideration to add that VS in the route table?