It’s pretty much irrelevant. You can stack containers vertically easily, and height difference doesn’t matter that much (8’6 vs 9’6 are the two common sizes). Below deck you have a hard cap because of hatch covers, but above deck you’re only really capped by sight lines. Lengths are 20ft and 40ft, so differ much more, and so having TEU or FEU as measurement is much more useful.
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It’s pretty much irrelevant. You can stack containers vertically easily, and height difference doesn’t matter that much (8’6 vs 9’6 are the two common sizes). Below deck you have a hard cap because of hatch covers, but above deck you’re only really capped by sight lines. Lengths are 20ft and 40ft, so differ much more, and so having TEU or FEU as measurement is much more useful.