The purpose of waiting station is threefold. First, it improves the efficiency of the operation by having the approach ship begin coming alongside from a fairly close station, shorter approach times mean less waiting around on deck. Second, it provides the approach ship an opportunity to gauge the guide ship's course and speed accurately. Thirdly, it gives everyone on the bridge, an opportunity to acclimate to being at such close proximity to another ship. All ship handling on the approach ship side is relative to what the guide ship is doing, so matching course and speed is critical. A waiting station is usually 600 yards astern the guide ship, just outside the guide ship's wake on the appropriate side, with about 100 feet of open water between the approach ship's side and the guide's wake. Ships normally spend at least ten minutes in waiting station, and may spend 30 minutes to an hour if one arrives early.
When the guide ship is ready to receive the customer ship alongside, she'll indicate that by hauling up the Romeo flag on the appropriate side. At that time or whenever ready, the customer ship will commence her approach alongside the guide. The approach ship indicates the commencement of her "approach" by also hauling up the Romeo flag on the appropriate side.
An UNREP demands the very best of helmsmanship from both the guide and approach ships since, as the two ships close each other, the hydrodynamic forces will both change and increase noticeably. At a replenishment speed of 12 knots, a one-degree course variation will move the ship 20 feet sideways per minute. The best separation alongside during the replenishment depends on a number of factors, but is controlled by wanting to ensure the safest separation while keeping the probes seated. For surface combatants, 140-160 feet seems to work well. Larger ships seem to favor 160-180 feet. Carriers are especially challenging because of the flight deck overhang, but by the time the separation increases to 200 feet, they are probably at the point of unseating the probes.