Nearest
Send the vehicle nearest the pickup, measured along the guide path.
The default from this phase on, and the one a modeller expects when they think "send the closest cart". Distance is never straight-line separation: on a one-way loop a vehicle a few feet past the pickup point has to travel all the way round, so a Euclidean rule would send precisely the wrong one -- quietly, with no symptom other than a fleet that performs worse than it should and no indication of why.
Unreachable vehicles are excluded rather than ranked last. DispatchContext.distanceTo reports an unreachable location as an infinite distance, which would sort correctly but would also make an unreachable vehicle win whenever it is the only candidate -- and it would then be assigned a task it cannot begin.