flash = the artist's own design they want to realize on skin
usually flash designs are similar to the work you see in the artist's portfolio and you can pick your flash on the day of your appointment. Some artists show flash before, some don't, always depending on the artist.
Same with repeating designs, most artists don't repeat them, some do.
Mella, Lara and Biel do their designs just once so one can be sure to receive a unique piece.