One request, several rails, and two ways to divide it
When more than one rail is open, the operator has to decide how much of the request each one carries. Two rules are in common use, they produce different payments for the same request, and the difference is <strong>18.00</strong> on the desk’s second sample.
- Waterfall
- fill one rail, then the next
- Proportional
- each rail pays its share
- The difference on S2
- 18.00 moves
- The sample’s default
- waterfall, oldest first
The two rules
Both rules are bounded by the caps and both are published somewhere in the operator’s terms, usually in one sentence that most readers pass over. The difference only appears when a request is smaller than the total cap and more than one rail is open.
Waterfall - fill one, then the next
The first rail in the published order is paid up to its cap, then the next, and so on until the request is met. Most requests touch one rail. A rail that is first in the order is retired fastest, and a rail that is last may never be reached.
Proportional - each rail pays its share
Every rail pays the same fraction of the request as its share of the deposits. Every rail is touched on every withdrawal, every rail’s cap falls together, and no rail is retired before the others - which spreads the closures rather than concentrating them.
Both columns total 40.00 and both respect every cap, so neither is wrong. What differs is which rail is left with capacity afterwards. Under the waterfall the card is spent and the wallet keeps 60.00; under the proportional rule the card keeps 18.00 and the wallet keeps 42.00. On a request this small the difference is 18.00; on a request near the total cap the two rules converge, because there is only one way to spend every rail completely.
Which numbers the rule changes
Three things move when the rule changes, and all three are things a reader notices.
The payments
A waterfall tends to produce one payment for most requests and several only on large ones. A proportional rule produces one payment per open rail on nearly every request, so a reader with four rails who ever withdraws a modest amount sees four credits each time.
The speed
An e-wallet can complete in hours where a card takes working days, so a proportional rule multiplies the slowest rail’s timeline across every withdrawal instead of confining it to the requests that reach it. Under a waterfall, a slow rail that sits last is rarely used at all.
Finding out which rule applies
The rule is written in the withdrawal section of the terms and it is usually one sentence. Where it is not written at all, the payments themselves are the evidence, and the test is simple.
- Make one withdrawal that is smaller than the total cap and that touches more than one rail in principle.
- Count the credits. One credit means a waterfall; one credit per open rail means a proportional rule.
- Compare the amounts against each rail’s share of the deposits. Shares mean proportional; a full cap plus a remainder means a waterfall.
- Note which rail snapped shut. The rail that reaches zero is the front of the waterfall order, which is the other half of the answer.
- Write the two things down. The rule moves money between rails and the order decides which rail is retired, and both are stable enough to plan around once known.