Ahhh ok !!!! 
Pff I'm using it the wrong way from the beginning !!! 

If I understand well (adding what is written here) :  http://www.shinken-monitoring.org/wiki/setup_dependencies_in_shinken

- Tag Business impact on End User Services  (for example Web Application, ERP ...)
- Define theses EndUserServices as BP Rule or/and with IT Services dependencies ( ie : db instance ...)
-- I think BP rule is more shinken way :p
- If there's is a problem on EndUserServices, shinken will compute the root causes and notify for the right service
-- If the db is down, no need to notify for Web App (despite hight business_impact ?) but only db service
- By the way, in Web UI WebApp will be showed as Critical so will Db server !

As Business_impact is on EndUserServices, the same IT service (ie db server) can be sometimes Critical (for ERP) and sometime be with low priority (same db server for a web app in development)

Thanks for your explanations.