Yeah, the implementation is pretty simple right now: similar to a thin, smooth glass, where reflections don’t disappear at 100% transmission like they would with alpha transparency. See discussion in MeshPhysicalMaterial: Rename .transparency → .transmission, add .transmissionMap by donmccurdy · Pull Request #19690 · mrdoob/three.js · GitHub.
We would like to support the full glTF transmission material spec (which would include surface roughness), and eventually volume, refraction, and subsurface scattering.