The apparent excess E-R capacity factor (E-R coupling control factor), jExR = (E-R)/E = 1-R/E, is an expression of the relative scope of increasing ROUTINE respiration in living cells by uncoupling. jExR = 0.0 for zero excess capacity when R=E; jExR = 1.0 for the maximum limit when R=0. The ROUTINE state of living cells is stimulated to ET-pathway by uncoupler titration, which yields the excess E-R capacity, ExR=E-R. Since ET-capacity is significantly higher than OXPHOS capacity in various cell types (as shown by cell ergometry), ExR or jExR is not a reserve capacity available for the cell to increase oxidative phosphorylation, but strictly a scope (reserve?) for uncoupling respiration. Similarly, the apparent excess E-P capacity, ExP=E-P, is not a respiratory reserve in the sense of oxidative phosphorylation.
Reference: Flux control factor
MitoPedia concepts: Respiratory control ratio