In the general context of the thread - fossil fuel is a general term for various resources and actions and things to create the parts that are needed for the Buff to be applied. Those parts are all based on whatever the Devs feel is best.
The basic idea is to remove guild buffs from the Coin requirement and into a requirement design that uses resources from planets and actions from guild members. This moves the guild buffs into a better model that all levels of people can use without the hard link to coin which is pretty hard for smaller guilds or casual ones. Instead it is your actions and resources.