As far as i know and referred descriptions its pseudo property which depends on the implmentation means, take a example
1. If you are runing a search for page where title contains "xxx" then jcr:score will always be calculated when you use "contains" function in your query and actually jcr:score gives the proxymity of those pages matches your condition (that is your specific implementation)
Now another example
2. If you are running a search for pages let say based on modified date then in that case the jcr:score will have some value but that is not supportive to your search as you already searched pages based on modified date and its already sorted but still jcr:score will consist some value while be used not by you but by system when again same search runs.
So mainly its a property which defaul associates some time its give some sense (example one) where you can actually find some logical connnection and some time not (like second example)
I also suggest you to just refer (May be you already referred)- http://www.day.com/specs/jcr/1.0/220.127.116.11_Property_Constraint.html, http://www.day.com/specs/jcr/1.0/18.104.22.168_jcr_contains_Function.html
What is your exact use case? Please refer  for scoring. Many times configure boost  as per your project need will help.