canProject

Whether this segment's column can give values back exactly as the documents wrote them.

false for a column written before the fidelity flag existed, and false for a numeric column whose values do not all carry its common scale. Both mean the same thing to a caller: read the document.

Whether ordinal's value at path can be read without opening its document.

Three things have to hold: the column reconstructs exactly, the ordinal is not residual — the column did not store it, so only the document knows — and it has at most one value, because a projection names one location and a path with several is not one a projection can spell.