Direct coaddition involves applying a relative astrometric solution (as produced by the Single Frame Pipeline, 02C.03.01) to input images, warping them to a common coordinate system, and coadding the pixels. This includes generating a combined PSF model based on the inputs.
This functionality is required by LDM-151 §5.2.1. It is described by LDM-151 §§126.96.36.199 & 188.8.131.52.
Per this milestone, it is possible to generate direct coadds but we impose no requirements as to computational performance or algorithmic fidelity.
In practice, this is a port of existing functionality from HSC, which is itself mostly just a refactoring of the existing LSST pipeline with relatively minor algorithmic improvements.