Every building chain that unlocks a Hero has the a building providing a " Recruitment Rank" bonuses.
(The building is usually placed 1 or 2 spots after the one that unlocks the actual hero recruitment)
It is present for Shamans, River Troll Hags and Goblin Big Bosses.
But it's missing for the "Black Orc Barracks" (lvl.4) building. See screenshot.