A lot of years ago, maybe when you were child, there was a widely used technology called Flash, developed by Adobe.

Flash was so popular, because it was one of the first technologies used to create dynamic websites without a lot of work. Actually, a graphic designer was able to create an interactive website using Flash, without knowledge of programming; and at the same time, Adobe provided a scripting language called ActionScript to create programming routines for advances users.

As you can imagine, Flash was used basically for frontend purposes. Without requiring knowledge of programming and being so easy for inexperienced users to use to create forms, dynamic websites, and routines. Websites designed with Flash had a lot of problems with the lack of input validations. A XSS's found in Flash websites are called Flash-based XSS.

But, as you can infer, actually these XSSes are common XSSes.