Explain the standards that need to be met before the flags can be ordered to half mast after a death.

We would like a definition as to what constitutes the flags to be flown at half mast. Celebrities die (Whitney Houston, eg) and President Obama orders the flags to be flown at half mast but four servicemen die in a local shooting (Chattanooga, Tn) but the flags are not lowered. This has happened several times in recent history. What makes a celebrity more important than the servicemen who gave there lives protecting our freedoms? We as Americans deserve an answer to this question.