Any resource or condition can be an environmental variable. An organism within the limits of S will survive. An organism within the limits of M will survive and maintain itself. An organism within the limits of G will survive, maintain itself, and grow. And an organism within the limit of R will survive, maintain itself, grow, and reproduce.
Regulatory organisms require energy to regulate their internal environment.
A is the amount of energy needed to increase the variable internally when the external environment is low in some variable. B is the amount of energy needed to decrease the variable internally when the external environment is high in some variable. C is the point at which the external environment and internal environment are the same, therefore, no energy is needed in regulating the variable. Ecological niche is a concept - not a real entity. Niche is measured by determining which parts of each axis describe conditions, resources or roles are actually or potentially available to that organism.