A macro the will equate to the currently played characters level so it can be used in filters so for instance you could do
NPC:1;MyLevel # All NPC Mobs that are my level
or Maybe
NPC:1;(MyLevel-3)-(MyLevel+1) # All NPC Mobs that are between 3 levels below and 1 level above me.
Alternativly consider macros so you could do
NPC:1;BlueCon, WhiteCon, YellowCon # All NPC Mobs that con blue, white and Yellow.
This would increase the usefullness of filter files across multiple characters and allow better customization of the category manager.