Publication Type
Since Apple's iOS 11 release, iTunes / Apple Podcasts supports new RSS tags. The new tags include the tags itunes: type.
There are 2 possible ways to publish a podcast. "Episodic" and "Serial".
Episodic is the classic kind of podcast where the latest episode always appears first in the feed. Serial is a new variant that allows you to define seasons.
This field can not be changed after the podcast is published.
More information about the new iTunes tags can be found here:
Http://podcasts.apple.com/resources/spec/ApplePodcastsSpecUpdatesiOS11.pdf