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We expect this forward-reference feature evolved out of a reasonable pattern of behavior for the way a successful data format evolves. A data record layout existed. The format is mostly fixed-length required fields. That layout cannot be modified. However, it needed to be extended to accommodate more fields, and those are added onto the end of the data layout. However, there was an opportunity to save space by reusing some preceding areas of the record layout that would otherwise be unused in certain new uses of the now-extended data record. The flag to indicate this reuse is of course a new field, and must be added on the end of the record. Introducing a choice earlier in the pre-existing part of the data record layout is allowed. It doesn't change anything about the existing layout, as the first branch of the choice would contain the pre-existing layout. Other branches would contain the re-purposing of that same data area for the new extended purposes. All this naturally leads to a choice (for the reused parts of the pre-existing data record layout) where the flag appears after the choice at the end of the record.
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