What does the icon for the unit type indicate on the hero page?

here are 4 icons. The book icon means it is an Economy hero and doesn't have troop boost skills. The other 3 specify that the hero is a military type and will boost the stats of 1 of the 3 troop types. The Shooter (gun), Fighter (Axe) or Rider (Motorbike).

In each case the heroes Troop skills will give bonuses to the DEF, HP and ATK stats of that particular type of troops when you use it in an attack.