Im almost postive its random. See as if it wasent we could just buy for marks. Also just a tip get vangusrd 3 on all characters. Then get all the others to 3 on the character u play pvp with. This way you dont have to earn cruciable marks on 3 diff characters. And i agree some are not used to some mmo aspects. But bungie did make it very obvious from day one where to look for rewards. Hint hint and the marks per week cap.