.in formal writing, before a coordinating conjunction, although this comma is optional if the coordinate clauses are short; when an element at the end of the sentence offers a shift in the idea of...

.in formal writing, before a coordinating conjunction, although this comma is optional if the coordinate clauses are short;

when an element at the end of the sentence offers a shift in the idea of what’s gone before or is an aside that’s not essential to the meaning of the main clause (end-of-sentence modifier);