Can "loss of life" abilities be triggered by combat and/or direct damage?
I'm not sure to which card you're referring, but consider the wording of Archfiend of Despair:


  At the beginning of each end step, each opponent loses life equal to the life that player lost this turn. (Damage causes loss of life.)


So combat damage and direct damage do cause loss of life. One of the main differences is that there are several cards preventing combat and/or direct damage, but only very few prevent loss of life (usually worded as 'your life total can't change').