The internet is a weird place ok. And don’t make it too ridiculous; a flat multiplier is an oxymoron.
Proper uses are where the increase is static no matter what
Flat damage additions (ie. Concentrate) or attack increases (raw attack bonuses, like what pommel beat and double slash get on stunned targets)
Flat increases of an otherwise derivative stat (ie. Sneak Attack for crit rate%, when crit rate% is a derivative of your crit rate number and the enemy’s crit resist)
Anyone using it otherwise is flat out (kek) wrong.
But you got mad about an instance of the second one which is what prompted me to reply to you that he was actually using it right in that context.
Just for fun, if someone were to say something stupid like ‘it adds a flat 5% to your damage!’ then unless you take your damage as 100% and whatever they’re talking about makes your damage ALWAYS 105% (which would, accurately, be a flat increase, being a static increase on a ‘derivative’ stat (the entire damage formula), but this is really stretching definitions here), they are using it horribly wrong and need to be burned at the nearest stake.
And because I love jumping into conversations halfway through without reading other replies, now I do have to ask; Does Prana give pyschokinesis skills an actual flat 50% increase (as in, if the skill does 250%, does it do 300% with Prana?) or is it a 50% modifier that multiplies the % skill factor in the new formula? The latter would definitely be the wrong time to use the word flat; the former is correct usage.