In general, a subscription is a viewer paying to access something as much as they want over a fixed amount of time. Pay Per View, on the other hand, is someone paying directly to access something. For example, someone can set up an annual subscription where viewers can watch uploaded content as much as they like over that year. While someone else can set up a pay-per-view for a 2 hours program where someone gets to watch only 2 hours of content and nothing more.
DaCast supports both models.