First, we can discuss the compulsory labor draft of the ruled class. The ruled class in Koryo dynasty included commoners, Hyang, So, Pugokmin, slaves, and the like.
Commoners were the main objects of exploitation of the feudal state, of which majority was peasants, and there were other classes of handicraftsmen and "ower" laborers.
Hyang, So, and Pugokmin were the lower laborers treated discriminatingly by the feudal rulers than commoners, and they were imposable objects of the compulsory labor draft.
Slaves were the lowest-class people and they were classified into public, private, wandering slaves, etc. according to their belonging.
Next, let's discuss the compulsory labor draft of the ruling class. The ruling class in Koryo dynasty included the royal family members, aristocrats, local officers, Buddhist priests, etc.
The royal family members, the highest privileged class, were exempted from all burdens of state, including the compulsory labor draft, even when the relations with the king were very distant.
Aristocrats had various social privileges including ones to become a bureaucrat of a feudal state, and most of them were the feudal landlords with a large area of land and slaves who exploited and oppressed the popular masses.
Local officers were the native public officials of the country and exploiters who directly carried out feudal rule of the state.
Buddhist priests were not the objects of compulsory labour draft and received the special treatment from the feudal state of Koryo dynasty, because the feudal state established Buddhism as its main ruling idea.
Owing to the reason that the priests were given the preferential treatment from the feudal state, so many commoners, slaves and even local officers tried to become monks to get rid of feudal exploitation including compulsory labor draft.