The lowermost point at which a cash flow can be calculated. That means it must have a time-series forecast of one or more of CapEx, OpEx, Production or Revenue.  This may represent a well-zone in some regimes, and it may represent an entire PSC in others.  Other examples might be an operating facility (cost only, or cost and revenue), an R&D investment (CapEx only), a processing fee contract (fee revenue), or a head office G&A burden (cost only).