RSS tag <itunes:type>
There are 2 possible ways to publish a podcast. "Episodic" and "Serial".
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.
Since Apple's iOS 11 release, iTunes / Apple Podcasts supports new RSS tags like <itunes:type>.
Further details on the specification can be found at PodcastConnect.