Good morning

In the handbook for COSMOtherm page 203 , they explain the following about the calculation of sigma potential similarity:

COSMOtherm allows the calculation of a solute-specific sigma-potential similarity 𝑆𝑖,𝑗 𝑆𝑃𝑆(𝑝𝑘(𝜎)). In this variation, the sigma-potential similarity for compounds 𝑖 and 𝑗 is weighted by the sigma-profile 𝑝𝑘(𝜎) of compound 𝑘.

Who is k? The solute? The solvent? When I try to make the calculations withouth weighting any compound, weighting the solute or weighting the solvent, the results are completely different. I am a little lost about this thing. If someone can help me, I will appreciate a lot. Thank you so much.