i'm gonna post 3 pics.
a single hole. a double hole and a tripple hole.
obviously those with 1 hole leave you no choice what hole to use.
units with 2 or 3 holes give you the option to get more draft response using a taller hole. obviously you do not want a plow in the top most hole of say.. a 3 hole rocker as it will exert lots of leverage on the draft control. that's why many reccomend to use the lowest hole possible for tillage.
pics credit to: mr john smith