What does ionic charge mean in practice?
Each ion carries a charge depending on its molar concentration and valency, which allows you to prepare a balance of cations and anions.
When does it matter?
Resins exchange equivalents, not the same mass of ions, so divalent and monovalent ions load the filter bed differently.
Which solutions are related to it?
Concentrations are converted to mval/l or eq/l and the balance is checked before calculating the process capacity.