The assault game mode and releated modes are a combination of ini changes in a mod and a special mappack that contains maps with trigger systems that make up the majority of the maps special behaviour. The game mode was developed by Cannis and DeeZire as an attempt to create something like capture the flag.
The assault mode and releated modes center around the destruction of your opponents flag structure. If at any time you succeed in destroying an opponents flag, all of their units and buildings are instantly destroyed, removing them from active participation in the game.
There are a number of things to consider that make up the assault game modes and they can be broken down into ini changes and map changes.
There is now a flag at startup for each side. The objective is to destroy that flag then creating a chain reaction thus destroying everything the opponent owns.