A Serial is a story with multiple chapters.
A Series is a group or collection of independent stories with something in common.
How to decide between a serial and series?
It's simple. Ask yourself this question: Does the reader need to read all the parts to understand the whole thing? Or Can a reader read one part and know everything they need to know?
If a part of the tale requires a previous part, then it shouldn't be a series; it should be a serial and posted as multiple chapters.
If each part has a beginning, a middle and an end with no loose threads, then it's a story and should be posted as an independent story, and the whole should be organized into a series of stories using the series management tool on the site.