M corresponds to the species. These would include:
S indicates the lattice site that the species occupies. For instance, Ni might occupy a Cu site. In this case, M would be replaced by Ni and S would be replaced by Cu. The site may also be a lattice interstice. In this case the symbol 'i' is used.
C corresponds to the electric charge of the species relative to the site that it occupies. To continue the previous example, Ni often has the same valency as Cu, so the relative charge is zero. To indicate null charge, is used. A single indicates a single positive charge, while two would represent two positive charges. Finally, ' signifies a single negative charge, so two, would indicate a double negative charge.
= a nickel ion sitting on a copper lattice site, with neutral charge.
= a chlorine vacancy, with singular positive charge.
= a calcium interstitial ion, with double positive charge.
= an electron. A site isn't normally specified.