I'm guessing it means don't use him vs spirit affinity (bosses presumably) because he'll be prone to do weak hits and be unable to place his HP burn debuff. Hard to say with no context.

I think it means that because Mordecai "places" the debuff, and dont "hit", no matter the affinitiy matchup, he always is going to put the debuff on.

Can´t happen a weak hit (so no debuff activation), because he dont hit targets... just place the debuff and thats all.

I belive the idea goes that way...

This  is  correct.  He  "places"  and  doesn't  "attack"  so  theoretically  the  only  way  that  he  can't  place  the  debuff  is  if  it  is  resisted  or  blocked  because  of  not  being  able  to  weak  hit.

Ah, that's more likely.

So  i  am  assuming  that  sicia  even  though  she  "places"  hp  burn because  it  is  based  off  an  attack  affinity  matters?

To  Land  a  Debuff  on  an  Enemy  you   have  to  pass  3  Check  Points.

The  3  Check  points  are:

1.  Chance  %

2.  ACC  vs.  RESIST

3.  Hit  Type  AKA  Affinity Match  up  

-----

Chance  %  determines  if  a  hero  will  attempt  to  do  Debuff.

Think  of  it  like  a  Bow  &  Arrow

The  Chance  %  is  you  attempting to  pull  back  the  Bow  to  release  an  Arrow.

If  Chance  %  is  below  100,  you  will  fail  to  pull  back  the  Bow  a  certain  amount  of  times

If  Chance  %  is  100,  you  will  successfully  pull  back  the  Bow  +  release  the  Arrow.

-----

ACC  vs.  RESIST

Previously,  you  was  attempting to  shoot  an  Arrow.

At  this  stage,  The  Arrow  is  already  fired  and  is  traveling  through  the  Air.

The  question is  whether  or  not  the  Arrow (Debuff) will  hit  the  target.

Accuracy  is  your  heroes  ability to  Hit  the  Target

Resist  is  your  opponent heroes  ability  to  Dodge  the  Arrow.

-----

Hit  Type  is  the  final  step.

It  is  the  force  you  used  to  try  and  pentrate  the  target  with  the  tip  of  the  arrow.

If  you  pulled  the  string  back  very  far,  It  will  release the  arrow  almost  like  a  bullet.

If  you  pulled  the  string  back  barely  at  all,  It  will  release  the  arrow  and  bounce  back.

It  will  hit  the  target  and  bounce  off  the  target.

It  will  not  penetrate.

We  call  it  Hit  types:

-  Critical  Hit

-  Strong  Hit

-  Normal  Hit

-  Weak  Hit

If  it  is  Critical,  Strong,  or  Normal  hit,  it  will  penetrate.

If  it  is  Weak  hit,  it  will  bounce  off.

thanks  for  the  clarity  on the  mechanics.    My  confusion  comes  from  hell  hades  commentaty.    For  mordecai  he  makes  the  comment  from  my  original  post  however  for  sicia  he  says  her  weakness  on  spider  20  is  affinity.    

both  "place"  hp  burn  so  i  am  unclear  on  why  affinity  is  an  issue  for  sicia  and  not  mordecai ,  other  than  mordecai  isnt  actually  attacking  To  place  his  burn

Correct. Affinity can be ignored for abilities that place debuffs without hitting, because you can't have a weak hit if you don't hit at all. Sicia does hit on her ability, so if she weak hits against Spirit affinity, she won't place the HP burn. 

She  places  it,  yes,  but  on  the  attack,  which  is  what  makes  her  able  to  weak  hit.