Server owners can register their own custom items with their own modifiers. The modifier will apply if a player holds the item in their hand or in the case of armour wearing it. You can either set it to detect all items of a certain material (name or anything else doesn't matter) or only if the item has a specific CustomModelData. The CustomModelData setting allows server owners to create their own temperature effects for items added by a custom item plugin with resourcepack (like ItemsAdder). These plugins often add new types of armour (with a different texture) while the actual item is still something like leather boots. These items have CustomModelData so the client knows what texture to apply to them. This CustomModelData can also be used in RealisticSeasons to give these specific items their own temperature effect.
Custom temperature items don't necessarily need a CustomModelData. For example, you could add torches as a custom item and give the player +10°C if they're holding one. This allows your players to warm up by holding a torch.