Basically what it sounds like, any space station / colony will allow you to divert excess production to your own projects be they ships, weapons, miscellaneous ressources or even just credits
Surplus is a somewhat catch-all and nebulous ressource that allows to make use of the colony's facilities for free with the explanation that whatever you ask of the colony through theses means is produced from leftovers at virtually no cost
Surplus may only be used to obtain stuff the colony produces for free, further, this ressource builds over time as the colony thrives, as such stability is a key factor in its generation and/or cap
I figure thats one way to make establishing colonies actually enticing, at the moment its just a drain of ressources and a major pain in the ass as well as a unending source of headaches...
