Episodic and serial podcast types relate to how syndicators group and order your content. Episodic episodes are intended to be consumed without any specific order while serial episodes are intended to be consumed in sequential order.
Taking into consideration the type of content within your podcast will help you decide whether or not you should select serial or episodic.
Episodic
For podcasts that are not linear in their storytelling and not intended to be consumed in a specific order, you can select "episodic". Syndicators that make use of this field will display your episodes from newest to oldest. You then also have the option to organise your episodes into seasons which will present the newest season first as well as arrange the episodes from newest to oldest. Examples of formats that are conducive to the episodic organisation would include interviews, conversations, single-episode storytelling, or any type of stand-alone content.
Serial
Specifying "serial" will have the podcast syndicator arrange your episodes and seasons from oldest to newest. The same as mentioned above, you also have the option to organise your episodes into seasons, however, the serial type will arrange the content within your seasons from oldest to newest while displaying the newest season first. Serial-type podcasts are conducive to long-form storytelling, narratives, thematic seasons, etc.
The serial podcast type requires episodes to be numbered when organised into seasons.