It's a formalization of one-way asymmetrically encrypted message delivery, rooted in the standard ECC technique of doing an EDH exchange on the sender side and sending the receiver enough information to replicate it, that has been generalized to support pre-shared keys (for instance, as a PQC stopgap), PQC algorithms like ML-KEM, and (presumably) RSA-KEM.

It's a subset of what Noise defines, standardized and further parameterized.

The point of the RFC is to level up (and make consistent) future cryptographic designs from the IETF. It's not something you'd use directly.