Here, there are f -bonds between the orange (P +) and purple (-) species, denoted on the schematic explicitly. The summation is over all possible 'center' monomers s, with the function θ s,-= 1 only when the salt ion is closest to s and θ s,-= 0 otherwise. We will discuss this function in more detail when we describe the practical choices in calculating these terms, but for now we will contend that it is this function that specifies if a salt ion is 'paired' with a polyelectrolyte monomer s. The area contained within the three orange ρ-circles alludes to the emergence of a three-density distribution function associated with all three monomers, once the integrals over polymer degrees of freedom in the overall cluster expansion is performed. The diagram in this notation only accounts for the specific product of f -bonds, and not the integration over the particle degrees of freedom, which we will explicitly indicate. This will help us work with this formalism going forward. Finally, the superscript (3) indicates this is only the three f -bond contribution, omitting the one-and two-f -bond terms that contribute primarily to the ring diagrams in the RPA calculation.