Publication Type

RSS tag <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. 

Optionally, the episode numbers can be delivered in the feed. This causes some podcatchers to display this number at the beginning of the episode title.

Serial is a new variant that allows you to define seasons.
This field can not be changed after the podcast is published.

Since Apple's iOS 11 release, iTunes / Apple Podcasts supports new RSS tags like <itunes:type>.

More information about the new iTunes tags can be found here:
Http://podcasts.apple.com/resources/spec/ApplePodcastsSpecUpdatesiOS11.pdf

Further details on the specification can be found at PodcastConnect.