What the class is actually for is contested, and the disagreement is not a quibble about translation. On the older reading, preserved in the standard English versions, chatat is the sin offering: an individual has done wrong, brings an animal, and the wrong is forgiven. On the reading argued at length by Jacob Milgrom, the name is formed from a verb stem meaning to decontaminate rather than to sin, the offering is a purification offering, and what its blood cleanses is the sanctuary, not the offerer. On that account moral and ritual faults generate an impurity that travels to the sacred precinct and accumulates there, and the blood rite is the mechanism for removing it, which is why the gravity of the fault determines how deep into the building the blood is carried.
Several features of the text favour the second reading. The offering is required after childbirth and after a discharge, where no wrong has been done at all; the blood is applied to furniture rather than to people; and the graded penetration of the sanctuary tracks the standing of the offender in a way that a straightforward pardon would not require. Against it, the text also uses the ordinary verb of forgiveness in the same passages, and critics have argued that the sanctuary reading imports a systematic theory the Priestly writers do not state.
The atlas records the class under the purification reading because that is the dominant modern position, and records the disagreement rather than deciding it. Both camps are working from the same chapters. What separates them is whether the Priestly system is a coherent theory of contagion or a collection of provisions that later readers have tidied.