Oxford (Serial) Commas
The Oxford Commas check attempts to determine if items are in a list. It does that by setting a maximum number of words that would still be part of the same list. For instance, the sentence, ‘He went to the park, the playground and the swimming pool.’ has no Oxford commas and three words after the final ‘and’. The sentence, ‘He went to the park, the playground and then he went all the way home via the swimming pool.’ has eleven words after the final ‘and’. If the maximum number of words were set to eight, then the first instance would be found (three words) and the second (eleven words) would not. You can set that number of words in this box. Decrease the number to find fewer false positives (but miss more issues) or increase it to find more issues (but also more false positives).
See also
Video Tutorials: Settings for Numbers, Oxford Commas and Style Points