Multi-Rail vrs Single rail:
Basically a single rail unit has only one 12V line which is drawn by all 12V output lines. A multi-rail unit has more than one 12V line which generally would be accessed individually by each output line.
The benefits of single rail vrs multi-rail were originally that a single output line would be able to draw up to the rated max current, while multi-rail units would be limited to the individual line (single rail typically 40A while a multi-rail might be 4 X 12A).
I'm still a diehard single rail fan, but I believe higher-end multi-rail units these days have the ability to automatically switch to single rail operation if one rail tries to draw more than the normal rating.
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