There are two types of alerts; TA and RA.
TA - Traffic Advisory's just that, that only tell the pilot that there is a plane near by (noramlly within 5 miles in the enroute enviroment and 2 miles in the terminal). (TCAS I and higher feature)
RA - Resolution Advisory's tell the pilot that there is an aircraft nearby and suggest a course of action. Following a RA is a valid reason to leave an assigned position (altitude, heading, course, etc.). There are may types for RAs but that is the basic jist. (TCAS II and III feature)
All airlines are requires to have TCAS II or higher if they operate a plane over 33,000 lbs with more than 30 folks.
Using the scenario of what happened over France, the BA a/c leaves his FL390 rather rapidly, he could then be in conflict with another A/c?[:0]
Does the TCAS scan above and below as well?[?]
And how would the aircraft dirctley above know the BA aircraft was taking immediate evasive action?[?]
Hope you understand what I am trying to ask?[?]
Thanks