The four natural flow categories overlap in some of their attributes and differ in others. The following table and discussion are intended to provide clarification for those studying ENL.
The four natural flow categories are listed in the table at left.
As shown at far left, what the top three have in common is that they are all biological flows. They are therefore subject to environmental budgets, as indicated in the second column.
The last flow category contains nonrenewables, which are non-biological in nature and therefore not budgeted.
The bottom two flows also have something in common. As shown at far right, they are both resources that are used as inputs to production.
This is not true for wastes and habitat destruction, which are consequences of production (and consumption), but not part of the material flows ending in outputs.
As shown in the third column, resources are allocated — that is, they are assigned to specific outputs so as to maximize overall health.
Renewables are the only natural flow that falls into both categories: they are biological resources that are both budgeted and allocated. However, ENL deals with budgeted flows by allotting them to outputs based on the marginal health criterion. For renewables, therefore, allotment and allocation are synonymous: once allotment is complete, there are no additional allocation decisions to be made.
Another characteristic of renewables is that they can serve as final outputs, in which case they are subject to output distribution. Important examples are the foods that nourished humankind during its lengthy hunter-gatherer stage, and that still nourish much of our species today: fish, game, nuts, fruits, and edible roots. This attribute is not recorded in the table.