The is the fourth book of the Abhidhamma.
Just as a doctor would find it useful to know a person's blood group, body mass, age, ethnicity, or other characteristics before treating him/her, a skilled Dhamma teacher like the Buddha or Mahākassapa would be able to tailor his teaching to suit the individual. A lustful person might benefit from meditation on repulsiveness, while an angry person would be better to use loving-kindness to develop concentration.
As for ordinary teachers, we just have to make an educated guess, and try again if the first method doesn't work. If nothing works, we have to let go, move on, and hope that someone else can help that individual.
Ajahn Chah was once asked by his Thai disciples how he could train Westerners since he could not speak English. He replied, "I don't speak buffalo either, but I know how to train buffaloes." He would just observe them carefully, and see what they did and how they responded to different situations.
• • • • (Upasampadā: 24th June, 1979)