Interchangeable use of an esszett?

In lesson 1.6 of German Rocket Premium, esszett is explained as representing two "s"s. Additionally, the word "tschüss" is introduced. Why does one use "ss" as opposed to an esszett? Are there certain rules discussed further along in the course or can they be used interchangeably?

I meant lesson 1.5.

Hi Laura, Use an "ß" only after a long vowel or diphthong: A Maß (measure), schließen (to close), reißen (to crack), draußen (outdoors). After a short vowel always ss instead of ß: wissen (to know), Kuss (kiss), dass (the). Hope that makes sense Gruß Paul

yeah I am still confused cause on the second last lesson of premium plus (survival kit) does not match with what you are saying on the lessons of Platinum. E.g. In den sauren Apfel beissen (proverbs lesson- Rocket German Premium Plus) in den sauren Apfel beißen (all beginings are difficult- Rocket German Platinum) (Actually i have found more differences in writing from Rocket German Platinum but that was the first one that came to my mind). It that because the ß while writing for English speakers it substitutes also with a double "ss"? I ask cause my bf (he is German) substitutes for English speakers the umlaut "ü" for "ue" and maybe this is sort of the same case.

Hi Cat O, We fixed the mistake. If you find more differences you would like to have changed feel free to let us know. I apologize for the confusion. All the best Paul