The specular light model uses two color textures: a diffuse base texture and a specular map texture. The diffuse texture is the color you get from all directions (the light is diffuse, that is, spread out). This base texture is the same as you've always known and used in Doom. The specular texture is the extra color you get when light reflects on the surface. The specular level and shininess (variables you can type in the GLDEFS file) controls how spread out the reflection is and how strong it gets. This light model is the classical model you've seen most games use in approx 1998-2012.
Physically-based-rendering, or PBR for short, is a newer model that is more physically accurate. It is what newer engines uses, such Unreal Engine 4 and Unity. The textures looks better under varying light conditions compared to the more classic model, but also require more texturing work. Here's a link to a general description of PBR:
https://www.marmoset.co/posts/basic-the ... rendering/
I recommend you choose only one of the models for a project. The reason being that dynamic lights hitting surfaces of one type reacts slightly different than if hit by the other. If the light in a scene isn't consistent then you could end up with visuals much worse than if you either had not used any of them, or if you had sticked to just specular textures. I added support for both models because the older specular version is 'backwards compatible' with classic Doom lighting, while the PBR one is not. That means you can much better get away with having only specular textures defined for some of your materials. For PBR it is an all or nothing proposition.
