Got one to look at?
The fatter fins that the water passes through are "rows".
If the inlet and the outlet are one one end of the rad/core then the water comes in, goes through half the "rows" to the bottom tank, where it enters the other half the "rows" and flows to the tank with the outlet. That's two-pass - in, down, change direction, return, out.
If the inlet and outlet are attached to the header tanks at opposite ends, then the water comes in the inlet, goes into all the rows, travels to the other tank and exits through the outlet. That's single-pass - in, down, out.
The advantage for single pass is that it has less restriction to the water flow (twice as much cross-sectional area as a same-outside-dimension two-pass). Depending on your flow rate, this may or may not matter much. There's also an interplay between water flow rate and radiator effectiveness, but it's small (I just see slightly different numbers for the same radiator with different pumps, for instance) and I don't understand it that well...
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