One is the usual one of cash, or in this case gold bars. You gain gold from taxes and trade, or you might gain some in a diplomatic agreement. Gold is used to build various things, but more importantly to pay your armies. Almost any action you care to take, be it contesting a holding, creating a new holding, espionage, forming an alliance, or whatever, can be influenced by you spending regency points on tilting the odds for success in your favour. This is a clever system and allows for such things as a realm with an assassinated regent being in disarray from losing all its regency points.
Holdings are of four types - law, guilds, temples and sources.
Guilds are a means to trade, representing merchants or rogues, and trade is at its most profitable between provinces with differing terrain types. Other holding levels must be built through regent actions. The Gorgon goes walkabout. The inter-relationship of every realm is reflected by their diplomatic status, and that status can be affected by all manner of things. As Dave said in his main review, the replayability value is high as each realm has different relationships, strengths and weaknesses.