Most of the time, there will be a theme and/or common limitation/rule for participants to follow. (E.g. B&W Jam - only uses the color black and white, a jam about time. - implement a game mechanic which makes creative use of the properties of time)
OST Composing Jam is an online music competition where musicians and composers all around the globe are challenged to compose an original soundtrack of any style for an imaginary game, in just a little over than a week, based on a given theme.