A full time rider pays a monthly fare and is guaranteed a seat in that particular van, regardless of how often they ride. A part time, or alternate rider would pay a daily rate just for those days they actually ride, but would not necessarily be guaranteed a ride every day. A part time/alternate rider must be willing to make alternative arrangments if upon meeting their vanpool group, the 13 full time seats are already taken. You may check with a coordinator ahead of time to see how often they commute with a full van, and how often they have an open seat. They are typically able to give you a good guess, but again, as an alternate rider, there is no guarantee. It is up to the descretion of the coordinator of a vanpool group as to how many part time/alternate riders they have on their roster.