What is the difference between these spell types in Spell.dbc?

[namreeb]

Spell.dbc describes the effects of spells/auras. It has an aura name it calls SPELL_AURA_MOD_THREAT, but it also has two other types SPELL_AURA_ADD_FLAT_MODIFIER / SPELL_AURA_ADD_PCT_MODIFIER which can modify threat.

Can anyone explain to me what the distinction is between specifying a modification of threat with SPELL_AURA_MOD_THREAT versus SPELL_AURA_ADD_FLAT_MODIFIER / SPELL_AURA_ADD_PCT_MODIFIER using a misc value of SPELLMOD_THREAT (2 numerically)?

Thanks!