I don't think anyone's supposed to do a certain kind of performance. A lot of girls like to do songs from Broadway musicals because they tend to be catchy and upbeat and crowd pleasers. Some girls tend to do mainstream songs because they're comfortable with songs like that. A lot opt to do ballads because slower songs are what they like.

The bottom line is, choose whatever best fits you so the judges know that you're totally into what you're doing up there!