What is a strategy rating?
According to several forums I checked, strategy rating is an obsolete mechanism used in certain published scenarios that would typically give some kind of advantage (initiative, timing of reinforcements, etc.) to the side with the higher strategy rating:

http://www.dakkadakka.com/dakkaforum/posts/list/282685.page

http://forums.tauonline.org/tau/5898-strategy-rating.html