The biomass yield coefficient is: $Y_{X/S} = \dfrac{\Delta X}{\Delta S}$ where $\Delta X$ is biomass produced (g) and $\Delta S$ is substrate consumed (g). It indicates metabolic efficiency — how much biomass is generated per unit substrate. The observed yield is lower than the true yield because maintenance energy consumption ($m_S$) diverts substrate from growth: $\dfrac{1}{Y_{obs}} = \dfrac{1}{Y_{X/S}} + \dfrac{m_S}{\mu}$.