descriptions taken from: http://www.wowwiki.com/API_UnitThreatSituation
	nil = unit is not on any other unit's threat table.
	0 = not tanking anything.
	1 = not tanking anything, but have higher threat than tank on at least one unit.
	2 = insecurely tanking at least one unit, but not securely tanking anything.
	3 = securely tanking at least one unit. 
	
or if you prefer:

	descriptions taken from: http://wowprogramming.com/docs/api/UnitThreatSituation
	nil = Unit has no threat
	0 = Unit has less than 100% raw threat (default UI shows no indicator)
        1 = Unit has 100% or higher raw threat but isn't mobUnit's primary 
             target (default UI shows yellow indicator)
        2 = Unit is mobUnit's primary target, and another unit has 100% or 
             higher raw threat (default UI shows orange indicator)
        3 = Unit is mobUnit's primary target, and no other unit has 100% or
             higher raw threat (default UI shows red indicator)