Difference between S and I in the context of magnetic resonance experiments
The nomenclature you mean has nothing to do with the electron spin S, it is something completely different. In heteronuclear experiments with two spins it is common to call them $I$ and $S$, with $I$ being the more sensitive nucleus. They're still both spin 1/2 nuclei.

$I$ is one spin, and $S$ is a different spin in this nomenclature, with a scalar coupling between them.

This nomenclature is usually introduced with the INEPT transfer and experiments like HSQC or HMQC.