I think the types of fog are not, in one word, well defined.
I'm by no means an expert on the various fogs. But, other
than light fog, non-image exposure to light, I think they
are all of a chemical nature. That includes dichroic fog,
over active developer fog, fume fog, and likely one
or two more fogs.
There are many kinds of fog. One kind is emulsion fog, where development center is somehow created on grains, so that they develop in absence of light exposure. There are way too many causes for this to happen, and history of silver gelatin emulsion is in one sense fight against fog. Bad emulsions can have fog right out of the kettle (reacting vessel). Emulsion fog can be caused by continued chemical reaction (sensitization reaction) during storage, naturally present radiation, etc. Then the coated material can be fogged by inadvertent exposure to light. These kinds of fog hcannot be effectively overcome by adding bromide or any other antifoggant to the developer. IF the fog isn't too bad, they may give you false sense of killing the fog, but that is because development is not complete.
Fog can be caused by overactive developer. If the reduction potential of the developer is too close or above the energy level of the conduction band of the silver halide, electron can be injected to the valence band without latent image, and the grain will be developed whether exposed or not. This nonspecific development of grains is called developer fog, and it's a sign of poorly designed developer. But this developer fog can be intentionally caused in, for example, second developer in b&w reversal process. Unrestrained developers may not be good enough for this purpose, and fogging agent like dimethylaminoborane is used. Some labile sulfur compounds are also active chemicall fogging agents. Sodium sulfide, polysulfides, or thiourea in alkaline solution can do this. Film or paper can be exposed to gaseous hydrogen sulfide for same effect.
Dichroic fog is caused by a different mechanism. This is caused when there is a high concentration of soluble silver complex and reducing agent is present. The silver is very often formed where diffusion is limited, i.e., near the surface of the coated material.
Anyway, when fighting with fogs, it is very important to know what kind of fog it is, caused by what mechanism.