Specular-glossiness
The older PBR workflow that drives a surface with a diffuse color, a specular RGB color and a glossiness channel — more freedom, easier to break the physics.
Here the reflection's color (specular) is given straight as RGB, which lets an artist set an arbitrary F0 — handy for exotic materials, but also the quickest way to accidentally make a surface reflect more than it should. The glossiness channel is roughness inverted: a high value means smooth.
The workflow appeared early in tools like Substance and was added to glTF as the KHR_materials_pbrSpecularGlossiness extension, but Khronos deprecated it in favor of metallic-roughness. It lingers in older assets and in pipelines where artists want the extra control.