Episodic content needs to have the <episode-num> element with an appropriate “system” attribute. An example using the SxxExx system would be: <episode-num system="SxxExx">S03E08</episode-num>
There are other systems, including original-air-date: <episode-num system="original-air-date">2021-02-19 19:00</episode-num>
Or, using the xmltv_ns scheme: <episode-num system="xmltv_ns">2.7.</episode-num>
The xmltv_ns system uses a season.episode.part scheme, where each is zero-indexed. So, the above example is for a single part episode of season 3, episode 8.
Multiple different <episode-num> elements can be present for a single airing (provided they are different systems). The above examples were for the same airing of a program.